I mean, that's the point of the visual design, you have greek gods vs the monstrous hordes. It's not a political statement, it's just aesthetics meant to induce coolness. Not something to be taken as some sort of political claim.
It's not about where they came from, the setting is just a plot device to facilitate coolness. It's not about them being from the middle east, they were just a random group they could pick to act like the vampires or zombies or what have you. Vampires are usually from slavic backgrounds but the Romanians never act like film is racist towards them for that. Same deal here. Middle easterners don't get to be more special than everyone else and avoid having their ancestors used to cool effect as antagonists.
No I don't think anything is offensive innately period, and people choose to be offended due to something within them, and not the object itself.
Accurately, it's more like "it's not a literal representation of your people, it's a fictional over the top reimagining of them as cooler and weirder than they likely were, which may coinside with how the people they raped and pillaged throughout the middle east at the time may have thought of them in their magical thinking brains at the time".
300 is not historical, it takes a historical theme and uses it to make a fantasy tale. It's a pure fantasy story that uses historical themes to have some common culture relevance so it won't have to set up an isekai anime style parallel world for the first half hour of the film before the cool bros beat the weird monster hordes, cause it's an american film.