Hmm, are you on a 32-bit machine? The build is 64-bit only; I could set up a 32-bit-friendly build if that's the case.
In that case I've no idea why the luasteam.dll isn't working... I had a bug similar to yours during development and in that case it was an issue with one of the dll files being 32-bit instead of 64-bit.
Then again, the Itch version doesn't require steam-related assets, so in the next update I'll just remove that from the Itch build entirely, which should fix the issue anyway!