Almost all games worked without the Expansion Pak ("Pak" intentionally spelt without a 'c'). In fact there were only 3 released games that required it[1] and even some of those were still playable without it, you just lost some content (eg Perfect Dark).
If this Mario 64 mod requires the use of the Expansion Pak then it's definitely not typical of other N64 games and a point that, like the GP, I felt needed more emphasis rather than a passing remark that was very easy to miss.
I'm not taking anything away from the incredible work that developer has done though. It's impressive and really interesting to see. From a hacker perspective it makes total sense to use the Expansion Pak because most retro gamers who might be interested in this mod would likely already have one. But from an authenticity perspective, this is a little bit of a cheat.
[1] https://en.wikipedia.org/wiki/Nintendo_64_accessories#Expans...