Considering Half Blood Prince and Deathly Hallows are significantly worse in every way than the likes of Prisoner Of Azkahban and Order of the Phoenix, then yes, it is hard to believe such statement.
Same question and/or request to you. Precisely HOW are those two books worse than "Prisoner of Azkaban" - which is still my personal favorite of all seven, btw. I honestly want to know just why you feel the last two books are worse. Because evaluating these things is so subjective for the most part, I am always interested in what criteria folks use to make thees judgments.
Book six and seven changed in tone from the previous five books, nearly every character was written out of sorts, Dumbledore for example went from a powerful wizard who had just (at the end of book five) dueled with confidence against Voldemort to a weak old man with delusions of grandeur whos great plan for Harry to beat Voldemort was to tell him stories of Voldemorts past (which, indicently, showed that Voldemort was born evil, negating the message of early books where everything is a choice), Hermione after five books of barely noticing or bothering about Ron while being completely focused and into Harry (I could list volumes of such incidents but it would take forever, ask if you wish to know some specific ones I am refering to). Plot points that had been given a lot of time to develop in previous books were dropped and ignored, ie the Veil in which Sirius fell through. Go back and read the section of book five that focused on it and then see how it is never mentioned again, or the locked door in the department of mysteries, etc..
Then there were stupid plot points that were created in the final two books, Ron speaking Parseltongue to open the chamber of secrets, ignoring the fact that previously this was considered a very, very rare magical gift and not a learnable language, so if the first five books were still in force at that point, Ron making random hissing sounds would not have opened the chamber but resulted in Ron looking like Joey in the friends episode where he thinks he is speaking french. Do not get me started on the idiotic idea of side-along apparation and the amount of moments it made look retroactively stupid from previous books when it wasnt used, most importantly the fact that Harry's parents when attacked by Voldemort didnt simply grab Harry and side along out, instead James went out to attack Voldemort while telling Lilly to grab Harry and "run".
The "romance" was written horribly. suddenly, after years of not giving a rats ass about her, Harry suddenly had a "monster in his chest" and could think of nothing more than wanting to kiss Ginny. How Ron managed to "win" Hermione, not by truely changing from being someone who constantly was rude to her, but through nothing more than a book that told him how to act nicer when around women. Or the fact that Ron needed a personality change in the final book, where the author gave him many characteristics of Harry, making him braver (him saving Harry and then destroying the horocrux when the trio were questing), making him smarter (the Parseltongue incident, as stupid as it was, was supposedly Ron being smart to think of this) and so on.
Or how about Draco, a guy for most of the books growing eviler with every book, delighting when the Basalisk was attacking students and hoping that Hermione would be the next to be attacked (book two), how he delighted in the fact when he thought Buckbeak would be executed (book three), how he was happy and joked about Digory dieing (book four) how he joined the mini nazi league with Umbridge (book five), only for him to be "redeemed" in a very fan fiction way in the final book and the completely out of character moment of him crying and baring his sole to a female ghost..
The homage/"rip off" of the Lord of the Rings with the Horocrux quest in book seven which was badly written, or the complete mess that was "wand mastery", which of course wasnt around in book five when the DA were dueling each other and regularly disarming each other of their wands...
Oh and the crappines of the epilogue...
I could go on and on, but the point is that I found near everything in book six and seven, the style of writing, the actions of the characters, the plot points used, the dialogue, the pacing, to be truely terrible, especially when compared to genuinely good books like POA and OotP.
In fact the only thing I found to be good of either the last two boosk was that in book seven, when Rowling was busy cementing the final relationship pairings, Hary and Hermione still got the most romantic scenes of the book. something the author has actually commented on since (around the end of 2008), in an interview for the books Harry: A History, where she says that in the final book Harry and Hermione share scenes that Ron is simply not a part of, how they are very intense and how the relationshiop question "could have gone that way" (Harry and Hermione).
Edited to add: Oh, and another thing that contributed to my dislike, the fact that the Patronus Charm, described in previous books as "complex magic", which even fully qualified wizards could not necessarily pull off, suddenly beacme the standard way in which every wizard and his dog sent messages to other wizards.