It says nothing about disabling anyone's defenses against Dementors; it is a fait accompli, where Harry simply states that anti-dementor spells now exist and are being actively deployed. The Wizengamot can disable him if they want, but it will not change the situation (unless they know Harry is bluffing.)
And why disable one of the few wizards who -can- actually combat the dementors? The only reason the ministry used them in the first place was because there was no way to destroy them.
Actually, that's not the only(or even best) solution.
It's pointed out in a previous chapter that intimate knowledge of such spells disables the regular patronus. Which is Harry's only weapon at this point - that threat. He can't say "I can kill dementors" without making the threat because he'll become an obvious man behind the break-in. What will he do, threaten to destroy them? They'll just send his ass to jail. No, he needs some kind of threat to the wizengamot, which in this case would be to ruin their Patronus spell. However, that still won't...
EDIT: New discussion thread here.
This is a new thread to discuss Eliezer Yudkowsky's Harry Potter and the Methods of Rationality and anything related to it. With two chapters recently the previous thread has very quickly reached 500 comments. The latest chapter as of 17th March 2012 is Ch. 79.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author's Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven, eight, nine, ten.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: