The ORIGINAL source code is copyrighted, but decompiling does not give you the original source code. Decomp tools give you generic variable names like unsigned_int_4 and then it’s up to you to decipher what the purpose of the variable even is and give it a relevant name. So it’s virtually impossible you’ll get a character-for-character match to the original.
Also, decomps have different levels of accuracy. You may get something that is a perfect behavioral match, even though there’s differences in the instructions being run. You may get an instruction-perfect match but not a byte-perfect match between the binaries.
IANAL but this is what I’ve learned from following decomp projects on YouTube
catloaf@lemm.ee 1 day ago
Yeah. And all the big projects so far have required a “legally dumped” ROM as an input.
smeg@feddit.uk 1 day ago
I believe that’s to get the assets (i.e. the textures, character models, etc) which are still covered by copyright and so can’t be included in the decomp projects