What is the significance of "This Isn't" in porn parody titles?

It's simply a way to be able to search the full movie title without them [quite, though I'd check with a lawyer] breaking copyright.

As these days most people use internet searches rather than physically browsing disks on a shelf, it gives them better chance of a hit.
It's also a way of saving them having to think up anything vaguely clever or amusing.

In the old days, they used to have to come up with vaguely similar titles that would be recognisable to their intended audience..
Jurassic Poke
Saving Ryan's Privates
Flesh Gordon
Sex Family Robinson
...I think you get the idea.

Most likely it's a way to avoid copyright issues:

Porn companies are able to get away with strikingly similar parodies of existing IP either by cutting secret deals with the original producers or by convincing courts that their films can’t possibly be confused with said content. That can probably be achieved by slapping an “XXX” and some nudity on the cover of the packaging.