Interesting. I was on PlayStation so I only heard about N64 dev at lunch. Factor5 were much closer to Nintendo than we were, and also known to be a bit good. Plus SDKs change over time. Take PS1 for example. We didn’t get GTE register details and inline functions initially, just libraries, and the more open it became the more we wrote in assembly.
Probably more accurate to say the custom microcode tools were not given to third party developers. From what I remember, you had to list the microcode used on your submission form, and Nintendo checked for it in your ROM. Did some third parties make custom microcode? Highly probable, but I do not know how many were able to ship with it.
I have been in the situation of standing outside an after-hours pick-up only window at a McDonald’s in the UK, able to talk to the staff, but unable to order because they only accepted app orders and I only had access to the Canadian app.