IANAL, but - there's no way that can be true. Reverse engineering of machine code is generally forbidden except for very specific exemptions, like compatibility with other systems. Furthermore, even under such exemptions, you are still generally not allowed to share the results.
Even putting reverse engineering aside, they are still distributing code that was written by Nintendo (although meticulously transformed in a way that makes reading and modification much easier), and which still belongs to them.
What would have been legal is if either this was a black box reverse engineering effort, in which all code was written without ever looking at the original machine code, or releasing a tool which fully automatically decompiles the ROM (which is not included with the tool, and must be acquired separately, probably from dumping a cartridge).
(Had to research this subject a bit as part of my collaboration on https://github.com/Davidebyzero/Snipes)