Already covered it, mate:And the Geneva Convention doesn't exist in Star Wars so it can go eat a dick in the context of Fiction.
tl;dr version:And it goes without saying, the Geneva Convention doesn't apply to the galaxy far, far away.
But here's the thing:
In the old movies, Luke discards his light sabre instead of striking down Palpatine or his father. It is a clear victory of virtue over evil. The protagonists should be heroic and do "the right thing", even in the face of literally unrelenting evil.
On the other hand, "the right thing" is based on our morals and ethics. Sure, you could say they have different morals in Star Wars, but pragmatically speaking, since it is a story, it is inevitably going to be based on our understanding of what is right and wrong.
And here's the deal: Sending a detachement that pretends to be on a diplomatic mission, claims diplomatic immunity to negotiate with someone, just so that detachment can then use that diplomatic immunity as a cloaking mechanism and surprise attack the enemy is one of the worst things that anyone can do in a war. It is pretty much only outclassed by a holocaust-style genocide. This has to be something that is true for all halfway civilized people, since the ability to have negotiations, to have diplomatic immunity, is absolutely necessary. It is in no one's interest to abuse this basic rule, since it takes away your ability to ever negotiate again with anyone.
If you're the guy that will pretend to talk things through before suckerpunching your enemy, no one will ever stop pounding your face into the dirt, no matter how often you beg for mercy. And they'd be justified in doing so. That's why pretending to be on a diplomatic mission is inevitably a dick move by Poe.
Again: The argument that the Resistance does not have to play by the rules since the First Order does not play by the rules either does make sense... but the Resistance are the good guys, they have to embody certain virtues and values to be the good guys and pretending to have a desire for negotiations is not a trivial matter.
Star Wars is a story based on -our- moral principles. Pretending to be on a diplomatic mission to prepare a surprise attack is amoral. It is dishonorable, evil and it can and will backfire in the long run if you get the reputation of pretending to enter peaceful talks, just so you can slaughter the other side. Also, even though there is no Geneva Convention in Star Wars, there still have to be similar rules, since these emerge not out of some politician's whims but rather out of a diplomatic necessity.
Similar rules predate the Geneva Convention by centuries, if not millenia, in the past it relied on an honor system, which shows us how fundamental they are.
Therefore, Poe's actions are a dick move.
I think it has the same issue as TLJ though:There. A real, actual plan that DOES rely on Holdo not telling Poe shit.
Why doesn't Holdo just tell Poe? Under 4 eyes, nothing prevents her from sharing that plan. And if Poe stood by her side, going "Trust Holdo", that would put many soldiers at ease.