I think anyone with half a brain cell would realize at that point that the Crimson Knights were laying down a trap. They're at an advantageous position, but they brazenly show their supreme commander as defenseless to bait an army that has got none to spare and is already on the verge of collapse. It's stupid to think that they're doing anything other than laying down a trap, maybe they wouldn't guess the rationale behind it, but they should at least think that something is amiss with CK's movements. Especially since there are soldiers there who had had tons of battle experience before. Author is dumbing down everyone to make Ashton look good because he can't think of a legitimately good battle tactic. It's going to get old real soon.
For one, I'll say that this is clearly not meant to be a war strategy manga, so just focusing on simple deviations on tactics instead of the grand scheme is reasonable.
Furthermore, this takes place in fantasy feudal times, most people are sitting there because of heritage and not merit. To expound on that point, it's not as if they have been lacking strategies. In fact, Ashton has only been there for a few meetings and hasn't been able to talk much, or say anything, regardless. Most of the brasher commanders have just been frustrated due to the length of the war and feel confident that they could at least take that regiment down with all their forces.
A reasonable idea, especially since it was made clear this only just recently happened, as most were unaware.
The biggest point here is not that they didn't realize it was a trap, as the top 2 showed pretty well how unsurprisinged they were and instead used Ashton as a way to bring strategy to the table from another view. This one, as a strategist of an elite mobile group. Not only were the brash commanders willing to listen to him, but because he held knowledge only the 7th division did (about killing the general), all the pieces on why fell into place, so rather than just, it's obvious, they all understand why that trap was made which makes it a lot easier to figure out how to deal with it.
Such adorable squeaking from Olivia, as well!