IIRC from an earlier read about this, the code is from a disassembler.
Nintendo did (supposedly) use an emulator for the N64 games in Nintendo Online (for the Switch); so it's possible that they don't have the source (or maybe it had enough machine-specific code that doing a source port would be more effort).
Of course, if they can re-sell it as a subscription service they probably would want to avoid releasing the source anyway…