I'm super new to minecraft modding, so please forgive me if the answer to this is obvious, but is it possible to use this with the old mcmod.info style?

It is not. If you wanted to use Kotlin for 1.12.2, use Shadowfacts' Forgelin. For 1.13+, you would use a mods.toml. There's one that comes with the example mod from the MDK, so I would check that out.