Pretty sure whether it was intended as a joke or not, it's probably got to do with "everything which is a door (class?) is weldable," whether it makes logical sense, is funny, or not.
That being said I've seen really weird "welded door" syncing problems since the gunslinger patch.
I've joined a game on Farmhouse, and I think there was another map as well - can't remember which one, where there was a visible weld "metal" as if the door was welded, but the door model was actually open, but you couldn't go through it... and it wasn't possible for joiners (me) to unweld it, as well as other joiners (they were complaining in chat), which is pretty weird... the weld tool just didn't work on that floating "weld," but the players who had been there since the start of the map could unweld it just fine.
It felt like there was some syncing of "door open/close" states which didn't happen when joining an in-progress map.
As far as welding being useful in KF2, it still is, but the "meta" lately has been different, probably due to map design, probably due to new player base and just the "boredom" of welding - which was even there in KF1. I know when I got better I just didn't enjoy playing with welded doors because I could handle the streaming of specimens just fine and reducing that stream made the game way too predictable and boring, nevermind that endless annoying banging on the door... but it was okay for a few maps in certain spots, I guess - like one of the doors in the basement of bioticslab, or the side door at spawn...