how to make a high sierra installer usb on a M2 mac? (Or work around "the recovery server could not be contacted")

OP wrote " want to make a bootable USB installer for High Sierra for an old Mac laptop (model A1278) using my M2 laptop. "

Not possible, even if your were able to download the Full Version of macOS 10.13 High Sierra on the M2 Apple Silicon computer

This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer.

The devil is in the details Qualifying Computer

Notation: If the computer being used to perform this action is Too New or Too Old to qualify to run the version of macOS - this computer can’t be used.

Alternative is, to gain access to a Qualifying Apple Computer from a family member, friend or associate.

In the MrMacintosh article, you should be using Fix #3. I've heard users on this forum have success with Fix #3.

What is the exact model of this Mac? This is critical information to help you. If this Mac can run a later version of macOS, then try using Command + Option + R instead which may give you access to the last compatible online macOS installer for that Mac. Or you can try using Command + Option + Shift + R to access the online installer for the oldest OS still available for that Mac. Some Macs may only boot to the online installer for the OS which originally shipped with the Mac from the factory regardless of the keys used for booting.

If this Mac is compatible with macOS 10.11, then you can generally use Mac models from 2007 to 2015 to create a bootable macOS El Capitan USB installer. If we knew the exact model, we could provide details of other Macs that may be used for later installers.

The following Apple has instructions for creating a bootable macOS USB installer, but requires access to another Mac compatible with the OS installer you are creating......this is the only reliable method of creating bootable macOS USB installers:

Create a bootable installer for macOS - Apple Support

To determine/confirm which Apple hardware is compatible with various versions of macOS, you can use the information in the following article:

https://eshop.macsales.com/guides/Mac_OS_X_Compatibility

I can't use the built-in Remote install for High Sierra ("the recovery server could not be contacted"), which this URL describes and has a solution for, but it doesn't work for me

How does it not work for you? FWIW it worked for me a few months ago with the old Mac that supports High Sierra. The instructions must be followed to the letter so try again if it fails and maybe also reset SMC and especially PRAM. But I hope Apple would finally fix this.