I mean, yes, the artist is taking us for a ride over the chest thing. It is very apparent that the magical amount of binding Rui does isn't literal magic on our MC to make him not notice the large tracts of land. The web novel doesn't really make a point of Rui's chest beyond when it comes up in dialogue or to talk about the binding. He does notice it immediately when she arrives without the binding (as seen here), so the artist is mostly being lazy. I'm also pretty sure binding your chest is just expected of male guard students to seem more masculine, which is a requirement.
I haven't read much, but this "Stares at Rui's chest" doesn't happen. Or if it does, it is much later than the events this chapter takes place in. He does meet the two other male guard students, and they talk, but Rui's affection is whispered between them. He isn't made aware that Rui is at fault for keeping the entire population of the school at bay. This also happens after talk about the field trip, which hasn't occured yet.
Rui and Kuno also talk on video after events. We don't see that aside here either. That gives us a bit more context that is just missing. Like how Rui, Kuno, and a few others have a 'harem' plan setup. Or how none of them have revealed that they are actually girls yet (this was the day of Rui rushing to help at the convenience store).
We also get two descriptions of the other two actual male students in his class, which we can see in chapter 1. One of them has a buzz-cut, the other glasses. The buzz-cut isn't seen here. Though that might be a mistranslation of bowl cut, I guess? That, or Japan has a really weird understanding of the buzzcut.
But also, MC is reaching theoretical particle dense. This is the man who didn't wonder why men were guarding men. And that assumed Rui had padded her chest for weird reasons.