Whew! For a moment there I thought that was a promo spot for an actual crossover. But it's just a gag.
Webcomic crossovers are like those general "website awards" everyone used to give each other twelve years ago. That is, they were all the rage once but crossovers quickly played themselves out and pretty much no webcartoonists do them anymore. At most one might see the odd background cameo of someone else's character(s) in a single strip, but even those are getting less frequent. Particularly for those webcomics that have moved from gag-a-day to ongoing storylines with a large cast and backstory, having characters from other webcomics pop in, even in the background, just doesn't seem plausible.