What does it mean that a question was "Modified by Community"?
In the current case it said so as a suggested edit by somebody that has no account took effect.
Even without an account one can suggest edits, if this edit is approved there is no user-account to which it could be assigned, and thus it is assigned to the Community-user.
Similarly, it would say this if the modifying action was taken by an account that afterwards got deleted (at the time of writing you can see an example for this on this meta with the question Who is MESE for, exactly?; to see it in that case, look for it in the list of question, it is currently the third, on the question-thread itself it is not visible).
Another reason for "modified by Community" is when the post gets automatically "bumped" to give it renewed visibility while no actual action happened.