I've always seen Piltover as being a bit of a 'watering hole' -- sure it's right in the middle of all these conflicting nations, but it's also a powerful trade hub with a tremendous amount of wealth and influence so it's able to leverage neutrality.
Looking into the OG lore that is presumably still canon-ish, Piltover and Zaun came to be because the wealthy class of the original Shuriman city of Oshra Va'Zaun decided to blow a big hole in the isthmus and create a canal / sealock called the Sungate that allowed them to control trade between the entire eastern ocean and the Conqueror's Sea.
Only the explosives were a little too enthusiastic and collapsed the older, poorer part of town into the fissures, and the inhabitants were only spared by a miraculous intervention attributed to the goddess Janna. The wealthy built Piltover literally 'over' the mess, Piltover got its Progress and its Sungate and its control of trade but the Fissures created the Zaun we know today.
So it already had that massive power as not just a maritime port but the chokepoint of all east-west trade between three continents. We're seeing in real-time events how a smaller nation with control over an important maritime channel can bring pain to much larger geopolitical rivals >.> so I could certainly see even Noxus not wanting to openly invade this independent city state that is presumably moving a lot of Noxian trade and travel through on its own.
Not when Ambessa could plant the heir of her house in an important position and have her there passively generating wealth and prestige for House Medarda while keeping her very far from the warfronts that Noxus is actually fighting on! Consider Noxus' military fronts here:
Demacia - waaaay over west, geographically very far from Piltover, with all the independent kingdoms of Valoran in between forming a 'buffer zone'. Also, Demacia is militarily defensive and rarely strikes outside its territory except to avenge a direct attack or eliminate a direct threat (see; Dawhold/Frostheld). So it's never going to threaten Piltover.
Ionia - waaaay over there to the northeast, a nation of mostly-pacifists invaded and colonized by Noxus that only recently managed to kick them out. Also not interested in threatening Piltover.
Bilgewater - an independent city-state, almost entirely pirates. This one is the big threat to Piltover's sea trade but also not interested in directly invading it when it can gleefully chip at its merchant vessels for fun and piratical profit, yaarr!
Shurima - enormous sprawling continent that used to be a powerful empire, that completely collapsed in the Darkin/Void war and is now a collection of mostly unaffiliated city states and nomad tribes. Emperor Azir has recently returned from the dead and risen his lost empire from the sands but... wow the timeline on when that happened in relation to Arcane is a bit hazy and our boy Azir is gonna need some time to re-establish his empire and there's the Void and Xerath to contend with so it's a mess down there. He's probably not invading Piltover any time soon.
Ixtal - enormously powerful mage civilization hidden in the jungle. Extremely isolationist after the war that annihilated its old ally/rival Shurima, to the point that most of the rest of the world doesn't know Ixtal exists. And the Ixtalis are happy that way, the ruling caste has the populace convinced that every other civilization was wiped out in the ancient wars and they're the only people left. So...not a threat to Piltover---BUT---as of current-League-lore, both Noxus and Piltover are poking their noses into Ixtali territory and doing a bit of colonizer b.s, Piltover poaching jungle animals, and stealing mining resources and artifacts, Noxus setting up military bases. The Ixtalis are trying to ignore this for now, the only one who wants to act is Qiyana, but she's a firebrand and a powerful one and would really love to smack Pill-tover all the way out of her jungle.
The Freljord -- crazy viking-esque raider culture but they're also all the way up there and Noxus is in between Piltover and them.
I think Piltover is actually pretty safe -- from everyone but Noxus.
So the idea of Noxus as this kind of militarily powerful neighbour with a complicated unspoken "you play your part and you get to stay independent" dance with Piltover, almost a protection racket, would make total sense to me.
And of all the Councilors I can see Mel alone with her Noxian cultural background being fully and painfully aware of all the nuance of this, and a lot of the other councilors being arrogantly oblivious to just how precarious Piltovan independence really is.
TONS of room for good fanfic/storytelling there.