Suppose an answer says foo is bar because of fact #1.
User Hardy comments in good faith:
fact #1 is not true on days beginning with T, according to the famous Professor Wiser's work in the 1960s.
Presumably this is a constructive comment. If it's correct, it might improve the answer.
User Laurel then comments in good faith:
@Hardy, In 1977 Professer Wiser revised his opinion, and concluded that fact #1 is only not true on days beginning with Th.
Many users would suppose that too is a constructive comment, but based on various recent deletions, many SE moderators do not agree. These moderators argue that Laurel is commenting on a comment, which is in of itself a misuse of comments and therefore should be deleted. Hardy's incorrect comment will stand however.
(Both commentators provide accurate URL references for their cited facts.)
In what respect is a comment correcting an erroneous attempted improvement no less constructive, (if not more), than the initial comment?