OK boomer 
Is that line supposed to be funny or something? Because it isn't. It just shows a lack of understanding of or willingness to discuss the issues on the part of anyone who uses it.
Yes, well, as with all examples of beratement, anyone laughing is laughing
at the targeted individual, rather than with. Whether or not it is funny thus depends entirely on which side you more identify or sympathize with.
And yes, it is a dismissive statement which can easily be used to stifle what is otherwise a legitimate discussion. In this case, however, it was made in response to a blatant personal attack. When someone starts making personal attacks, I'd say it is entirely appropriate to be dismissive of them, as they have demonstrated an unwillingness to discuss the matter at hand in a civil manner.