Posts which are considered as spam are not related to the topic then it will be deleted when a mod see it or when someone report it.
His reply was related to the topic. He even briefly mentioned a flaw he found with the service he tested.
In general your posts will be deleted if you post generic replies or saying something which has been already told even in the same thread a million times already.
That didn't happen either. His post was the first and only post made in that particular thread.
Don't reply to in a mega thread
A post isn't necessarily bad/spam/off-topic just because it's in a megathread. If the problem is with the thread itself, then it's the thread that should be deleted or locked.
Avoid posting one liners just for the sake of increasing your post count
No arguments there. One liners and replies that don't move the discussion in the right direction should be avoided.
Avoid replying to a very old thread when it doesn't necessary
True, but there are also valid reasons why you could do it and where it would be deemed necessary.