A covering letter fails in one specific way, and adjectives are not it. It fails when every sentence in it would survive being sent to a different employer — passionate about the industry, drawn to your mission, keen to bring my skills to a dynamic team. None of that is untrue. All of it is unreadable, because the reader has seen it ninety times this week and it tells them nothing about whether to interview you.
So this prompt writes the letter and then attacks it. The swap test in section two quotes back any sentence that is portable between applications, which is a much harsher and more useful edit than "make it more compelling" — it converts a vague feeling that the letter is bland into a list of specific sentences to cut.
Section three is where the real improvement usually lives, and it is the part you have to do yourself. One concrete fact about their product, their market, or a problem they have visibly got, is worth more than any amount of rewriting. The prompt cannot look it up, but it can tell you what would be worth looking up.