“This” is fine as long as it isn’t the only content of the comment. It is simply a signal that the following text isn’t meant to contradict what they are replying to. It’s a simple shorthand for “I wholeheartedly agree, and furthermore …”
I find it irritating, but I can see your point.
As the poster who posted it, it irritates me but it saves a lot of typing and is a well understood idiom.