At this point I feel like the foreigners being friendly and the entire thing being a misunderstanding stole the crown of "being playboys" as the more common trope. I'd be more surprised if they were actually hitting on them lol
I guess it's the question of whether it's more important to the story that they actually be trying to pick up Kano & Suzune, or it's appropriate to have it be a misunderstanding.
The former, at least also makes the brother's apparent "freeze response" more understandable and less worthy of condemnation. He would look more silly, I'd argue, if it turned out the two dudes were just friendly and it was all a misunderstanding and they weren't hitting on the sisters. I wouldn't fault him given the appearance of things, but the lack of actual danger, even known after-the-fact, wouldn't do him any favors.
And, them actually being after the sisters means Yuito's reaction is fully warranted, rather than him stepping in where it technically might not have been necessary. And in light of
that, I'm inclined to maybe change my original stance, for the sake of his character arc.
That said - there's also the potential they turn violent in being rebuffed by Yuito. No reason to think they necessarily would, but they're presented as large and imposing; there's implied physical danger in just their appearance, which is what makes Yuito's course of action all the more noteworthy. But if they
were to turn violent toward him, well now he's likely just gonna get his ass kicked until someone/something else can intervene.
So, mostly depends on what the author's trying to make happen from this. Yuito looks like a hero, more so if they're the geunine article. But hopefully he doesn't have his face rearranged to boot.