Yes, it's nice that Riot designed a champion that actually has the muscle that her kit requires. From what I know about her and her kit, she seems like a champion I'll want to buy. But there's more than one reason why she's the first female juggernaut: that being the obvious one everyone gets pissed about, which is making all female champions thin and very curvy, but there's also science.
Sorry, but men are just generally physically more powerful. I'm not saying that women can't be strong, but the strongest men in the world are also the strongest humans in the world because of testosterone. Furthermore, there's 4 juggernauts. All of them are men. Two of them are human. The other two I guess are dudes, but not technically human.
https://en.wikipedia.org/wiki/List_of_world_records_in_Olympic_weightlifting#Women_.281988.E2.80.931992.29
https://en.wikipedia.org/wiki/Brian_Shaw_(strongman)
I tried to find a Wikipedia page on the world's strongest women but female bodybuilding isn't as powerful because women are kinda discouraged from that and stuff.
I wouldn't call Illaoi lean. She's frickin' huge. So much muscle. Like a female Braum. They both use blunt objects as weapons too.
Illaoi IS unrealistic. Just not in the "anorexic barbie-doll like appearance to satisfy the desires of 15-year old boys," way. Just like all LoL champions, she's unrealistic af. It's logical she's muscular since she's a juggernaut and stuff, but women are almost never that ridiculously muscular. In fact I see more women on a regular basis who look like the other female LoL champions than I see women who look like Illaoi. I see more men who look like Darius and Garen than I see women like Illaoi.
Technically she's the first female champion (unless we count Kindred although you're technically 2 entities) since Rek'Sai. The first (living) human champion in a long time. Since Jinx perhaps?
The fact is, honestly Illaoi is one of the most unrealistic living, 100% human champions in the game. It's good that Riot's releasing female champions with more than one or two usual body types, but don't call it realistic.