I am going to put on my pedantic hat...1. That Site says there's a volcano roller coaster rumor buzzing about. Well, we've already determined that it was a once long-ago idea resurrected recently by Jim Hill as the only source. So, no, it hasn't been buzzing about as if people in the know are talking about it. One person talked about it and then it resonated in the echo chamber.
2. That Site says that the Volcano Coaster probably morphed from the idea of Volcano Resort. AFAIK, it's the other way around.
3. The land behind PotC isn't completely usable according to @marni1971 because of sink holes. You have to build something smaller around the sink holes, so, that's not a likely spot.
4. The other land they propose is the spot where the Venetian hotel was supposed to go, but, AFAIK, it didn't get built because the ground there wasn't suitable.
So....
The land where the Venetian hotel was supposed to be built can be built on, it is just not ideal to do it.
As it was explained to me, that particular area has a very high water table that creates a condition on bearing piles called negative skin friction. Basically, friction piles get sucked into the ground over time.
There are ways to deal with this, but none of them are easy or cheep.