I think they were confusing the other 'hide my email' service that you can use with paid iCloud+ It specifically says it is available on iCloud+. Those create @icould addresses and forward to random @icloud address but not these @privaterelay.appleid.com addresses that the app ones use.
I started a $.99 /month account and that iCloud part opened up and i tested an address (which worked and forwarded to the address I have set (same as the address I have on the app ones)) but the other 'hide my email' for apps still doesn't work for me even with a paid account.
My one last hunch on why my tests aren't working to the app emails is that they filter to only allow email from the corresponding app or the developer but I don't have a way to get an email from one of those apps to test the addresses on demand.
Too bad Apple isn't responding on this.
I won't be using the Hide my address on any new apps. Seems like the only way to change my existing ones is to try contacting the companies I used them with to try to change to new address through their support (without breaking your existing accounts with them or orphaning those accounts)