An exception to the rule and one that exists in a Federation over a century from Discovery. And, as has already been mentioned, one that required an intervention from Sisko.
Janeway often made reference to how her era's Federation/Starfleet was more "enlightened" than Kirk's era (whilst she, all the while, acted similarly, at times...). Can you imagine a situation where a Kirk-era (or, rather, pre-Kirk) Starfleet would've made that kind of exception? And who would have been requesting it? How would it be done? I'm not saying that it's impossible, just that it's improbable. Even today, in the U.S. military, you don't have to be a citizen to join initially...you just have to be one to reenlist.
And Saru is a Commander. What kind of circumstances would have had to exist a decade or more prior to 2256 for this kind of exception to come about?
There's a story to be told here, I think. It might even be interesting. I just don't think they've thought it all out, as of yet.
