I actually agree that in principle even if you assume the argument to be true, you need to determine whether it is relevant. If it was just "comfortable" dress policy I think you would have a point (whether this point would be recognised by the creators of the test is a different issue--personally I doubt whether several creators in all their wisdom would solve each other's tests flawlessly, but thats a topic for a different rant)
That being said, here they talk about a "relaxed dress policy", which seemingly refers to a non-strict policy rather than "mandatory-relaxed-clothes policy". I do agree that this is awkward phrasing, and is by no means obvious, but that is the unfortunate reality of the test (luckily some tests given by law firms tend to be of higher quality).
Edit: a somewhat useful rule of thumb with watson glaser (especially if you belong to a group of people who tends to put the questions under heightened semantic scrutiny) is that the test is trying to trick you a bit less than you may think