The difference is that when hiring engineer #1 there's no technical infrastructure to which you'd onboard someone, and nobody who's really qualified to make a decent technical onboarding process - you're hiring a person to create this process (among others) from scratch, make the initial decisions themselves (instead of being told them during onboarding) and onboard others.
I don't see how engineer #1 or engineer #100 changes that.
When I onboard a senior engineer, they get a problem on day 1.