I just want to use this with some other mods that mysteriously skipped 1.21

That would be because 1.21.1 was a hotfix version for Minecraft. It made no significant changes to the code, and is therefore compatible with 99.9% of mods.
Only reason Interactic isn't denoted 1.21+ is because it never needed a patch since that release

Ah. Well, the Modrinth app is finicky about versioning, is there a way to just mark the existing files as 1.21.1 compatible? Not a huge issue, but it was going to made me switch mods because I didn’t know.

It's vaild in 1.21.1, but invalid in1.21.3/4. I tried to fork this project and update it by myself, but only to found that a nonexistent class named InteracticConfig was referenced, who can tell me what the hell is that and how the code get compiled through?