I'm thinking he meant more in terms of who builds a website that respects the "like Gecko" in the UA string as UA sniffing has been considered a sub par method for a while now.
I agree with "who does that" because I've never done UA sniffing so I had to look up the term "like Gecko" to even understand the context. As someone else has pointed out, that's for browsers that are not using the Gecko rendering engine. So essentially, everyone is trying to be "like Gecko" in one way or another.
YouTube is a particularly bad offender. Linked to their mobile site on a desktop? You get the ultra-basic WAP-like version, instead of the smartphone version or a redirect to the desktop version. Linked to their mobile site in Opera Mini? Yep, WAP-like version, despite the fact that Opera Mini would have no problems with the smartphone version.