...that's a sizeable fraction of Konoha's military power. Are we sure they're doing what Yuno says they're doing?
How many chuunin does Leaf have? Let's say she means three backing her up, including Akane. ('A couple' could mean two etc.) Let's say that they're an efficient party and can kill enough chakra beasts every four days to save a village. Three week adventure, five villages saved, two of them high-value: IMO it scans that that Leaf is willing to commit 1-3% of their chuunin to what smells like a B-rank mission. If Leaf reserves half its chuunin at any given time, and every B-rank mission takes a similar force, then they can have 20-50 such missions ongoing at once. That number is probably a little lower given that chuunin are going to be on C-rank missions with some regularity, but you'll have the occasional jonin take a B-rank. Those numbers all feel reasonable to me.
Wait, she's a full jounin? Presumably she was promoted before she went missing, so it happened at the age of what? Despite the systematic persecution of her? Hell, Naruto isn't even a jounin, and he's an outright s-ranker with the most sympathetic and beloved backstory in history.
An explanation equally-consistent with reality: the Hokage has declined to as-of-yet rank Yuno, because she's only recently moved from 'irreplacable diplomatic asset' to 'someone whose death would be regrettable but not of geopolitical consequence'. This mission reflects that he's politely asked her if she'd be willing to go murder some chakra beasts, and she couldn't pack her bags fast enough.
We also have no strong evidence that Isan has the concept of genin/chuunin/jonin - she might have left as an uncategorized ninja, or Isan's ranking system might not map clearly to genin/chuunin/jonin, if they even have such a system. They were isolated before the establishment of the Hidden Village system. Prior to the establishment of the Hidden Village system, my impression is that groups of ninja were clans, with no need for formal designations of rank (you know that you're better than your cousin and worse than your uncle) or maybe you'd get a mentor running around with their pupils - also no need for a formal designation of rank, because you're teacher or student. Moreover, they don't have the same tradition of conducting ranked missions as we do (they don't have clients, after all, so whether a mission is A or C doesn't matter, because they aren't really getting paid for it) so the only reason for them to have ranks is to adhere to a sense of tradition and provide some gating for knowledge - can't learn Wind Scythe until you're at least a Quartz, and you're only a Fluorite.
In fact, I wouldn't be surprised to find that Yuno believes that our system of ninja ranking is somehow barbarous ('you mean you only have three ranks? how do you figure out who sits next to each other at dinner?!') and is perfectly happy existing outside of it for as long as she can.
A final possibility: Asuma will eventually designate Yuno a genin. She will take the chuunin exams and demonstrate the might of Isan-nin by almost-but-not-quite disembowling literally everyone she comes across. It doesn't make sense to designate her a genin for any longer than absolutely necessary, as she's then, y'know, a genin, but he can't show her off to maximum effect in the chuunin exams until resource-sharing agreements (his ability to send Isan-nin on missions) are firmly in place and everyone's trying to figure out exactly how badass the average Isanese ninja is.
