Oh, there is no such limit.
The post you link to is quoting another poster who is quoting something they heard from someone else. It sounds like something that may have originated with someone hearing about a particular implementation detail of our anti-abuse systems but the game of telephone has transformed it into something that's just not true at all.
As described in my comment on that thread, we do have certain anti-abuse heuristics which might result in a 1015 error. It's very unusual for people to hit these in legitimate use. Have you seen such an error? 100k requests per user per month doesn't sound like it would come anywhere near being an issue (even with 10M users).
FWIW the heuristics are designed such that simply adding more users (who behave the same as existing users) shouldn't ever cause a problem.
