The cloak is said to be able to hide one from the gaze of death itself, but that doesn't mean that if the cloak hides you from something's gaze, that thing must be death.
Not as a logical deduction in the strict sense, but its so obvious that he would consider it, and since Dementors = death also fits well otherwise that answer is much more probable than that dementors are something else, coincidentally also blocked by the cloak (which Harry somehow figured out in another unrelated way), and hiding one from the gaze of death itself refers to yet something else that is so far inexplicable.
Would a person have to think about universally conquering death to cast a true patronus, or is it enough to imagine personally conquering it? If all it requires is the latter, I can certainly see Quirrelmort managing it.
Even if personally conquering it were enough it has to be a positive thought and Quirrel doesn't seem to think about life in particularly positive terms, merely preferring it over the alternative.
Considering that the true invisibility cloak is said to have a general ability to keep one hidden, rather than merely invisible, I don't see how the fact that it hides one from dementors counts as evidence for the idea that dementors are personifications of death. It's evidence that it's a really kickass invisibility cloak.
- This thread has run its course. You will find newer threads in the discussion section.
Another discussion thread - the fourth - has reached the (arbitrary?) 500 comments threshold, so it's time for a new thread for Eliezer Yudkowsky's widely-praised Harry Potter fanfic.
Most of the paratext and fan-made resources are listed on Mr. LessWrong's author page. There is also AdeleneDawner's collection of most of the previously-published Author's Notes.
Older threads: one, two, three, four. By tag.
Newer threads are in the Discussion section, starting from Part 6.
Spoiler policy as suggested by Unnamed and approved by Eliezer, me, and at least three other upmodders:
It would also be quite sensible and welcome to continue the practice of declaring at the top of your post which chapters you are about to discuss, especially for newly-published ones, so that people who haven't yet seen them can stop reading in time.