To these three, the animagus potion is such a blatant Voldemort calling-card that it screens off Harry's involvement. Not only that, they simply haven't thought of the idea that Voldemort and Harry are working together, so to them Voldemort's involvement exonerates Harry.
Voldemort's potion was just that good of a trick. The only people it could fail to work on were people who were plotting at too low a level, and they were fooled by the time-turner test.
And why test Harry's time tuner if the potion rules him out completely?
Update: Discussion has moved on to a new thread.
After 61 chapters of Harry Potter and the Methods of Rationality and 5 discussion threads with over 500 comments each, HPMOR discussion has graduated from the main page and moved into the Less Wrong discussion section (which seems like a more appropriate location). You can post all of your insights, speculation, and, well, discussion about Eliezer Yudkowsky's Harry Potter fanfic here.
Previous threads are available under the harry_potter tag on the main page (or: one, two, three, four, five); this and future threads will be found under the discussion section tag (since there is a separate tag system for the discussion section). See also the author page for (almost) all things HPMOR, and AdeleneDawner's Author's Notes archive for one thing that the author page is missing.
As a reminder, it's useful to indicate at the start of your comment which chapter you are commenting on. Time passes but your comment stays the same.
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: