It seems to me like a gesture that Reddit will probably appreciate (though I don't know whether they'd say so out loud, politeness probably requires them to say something like "Why, PG, good sir, you didn't have to do that!") and so I see no reason to question the truth of PG's stated motive.
Wasn't there something in Methods of Rationality about this? A discussion between Quirrel and Harry, if I recall correctly.
The relevant thing here is not whether pg's stated motive is consistent with his actions, let alone whether reddit will appreciate his actions, but whether the mind-states corresponding to that motive are the most probable of those that would result in his saying what he did.
A good reason to question his motive is the fact that HN and reddit are not necessarily substitutes for each other; another is the fact that he's well aware (as others have pointed out) of differences between the two communities that he wants to preserve.
