Remove a device from trust list but not from account?

Actually, yes those two actions are synonymous. I don’t know why your iPhone didn’t take that disconnect, but removing a device from your AppleID breaks its connection to iCloud. Since iCloud is the means of sending 2FA codes, when you sign out of iCloud, or remove from your AppleID on the account management web site, you again are telling that system to block codes and access to that device since it no longer trusted, and it will stay that way until you have it in hand and can sign back in (using a 2FA code received on one of your other trusted devices).

Signing into iCloud on a device is what associates that device with your AppleID. So all of these features - 2FA (uses encrypted iCloud push notifications for codes), Find My, activation lock - are all tied to an iCloud account which in turn is tied to the persons AppleID used with iCloud.

Removing from the trust list means removing from your AppleID. There is no way to have a device listed under your AppleID and not have it as a trusted device. By signing it into your ICloud account on that device, you are telling the AppleID authentication system that this is one of your devices and that it is thus “trusted” for your AppleID.

Two-factor authentication for Apple ID - Apple Support

“Trusted devices

A trusted device is an iPhone, iPad, or iPod touch with iOS 9 and later, or Mac with OS X El Capitan and later that you've already signed in to using two-factor authentication. It’s a device we know is yours and that can be used to verify your identity by displaying a verification code from Apple when you sign in on a different device or browser. An Apple Watch with watchOS 6 or later can receive verification codes when you sign in with your Apple ID, but cannot act as a trusted device for password resets. “

Gotcha now. What you’re referring to has to do with activation lock. If you remove the device from the AppleID management web site, that action ensures it cannot receive 2FA codes any longer. It also disables Apple Pay from that device and removes the cards in the wallet app (for that device only). But you don’t want a thief to be able to actually use it, so it remains (I suppose you could say, partially in a sense) linked to your iCloud account via the Find My service only. As long as it remains that way, activation lock remains in effect.

Removing it from Find My at www.icloud.com will break the remaining connection to iCloud specifically, thus disabling both Find My and activation lock. BTW, if a thief erases it in iTunes, you’ll loose tracking as well. But as long as the device shows in your Find My linked devices at www.icloud.com activation lock remains in effect and they cannot complete activation and use the erased device.

Both actions would require a 2FA code from a trusted device to sign back in if you had the device in your hands and wanted to later on.

FYI - those two actions used to accomplish the same thing. It was the move to requiring 2FA with AppleIDs for certain services (e.g. Apple Pay, messages in iCloud, etc) and use of trusted devices for those iCloud 2FA code push notifications that I believe led to the changes that leave Find My and activation lock in effect while still allowing you to un-trust a lost or stolen device.

Do you disagree with this part of what I said above: "If you remove from Apple ID, then you will not be able to sign in to any account that requires the use of your Apple ID?"

Michael Black is correct in this sense at least. If you remove from Apple ID, then you will not be able to sign in to any account that requires the use of your Apple ID. Debating the difference in the two is not very productive, tho a distinction might be made.

Actually, I don’t think that link even understands what they are talking about. An AppleID, by itself is not an account - it is an identifier. It is the personal identifier you create with Apple in order to make service accounts. but when you first create an AppleID using Apple’s web site to do that, that AppleID doesn’t “do” anything for you, until you sign into an Apple service with it and create an account for that service.

An iCloud account is a service account you make with an AppleID. Yes, an iCloud email address is also de facto an AppleID (or an alias of your AppleID if you use a third party email address as your AppleID). But iCloud is an Apple service with a number of optional features.

Facetime, iMessage, the App Store, etc are also all Apple service accounts that you create by signing into those services with your Apple identifier, your AppleID. Each AppleID thus creates one account for each Apple service you use that AppleID with.

But an AppleID by itself is not useful for anything. it is only useful when used to create and use an account for an Apple service. You cannot add a device to your AppleID information without having an iCloud account as it is the act of signing that device into iCloud that associates the device with those trusted for use with your AppleID.

So a lot is semantics, sure, but I also find it helpful. I think of my AppleID as nothing more than a login ID or identifier. It is the services I use with my AppleID that are my Apple accounts, not the AppleID by itself. And it is the iCloud service account identified by my AppleID that ties my devices to the security features like Find My, activation lock, and 2 factor authentication.