Disney's VGF2 announcement was explicit:
Projected to open in summer 2022, these new Villas are an expansion of The Villas at Disney’s Grand Floridian Resort and will be a part of its current condominium association.
There is no such crystal clear statement in Disney's Poly announcement.
As to why not have a completely different themed resort, it's clear from Disney's announcement that they are focused on a Seven Seas Lagoon location, and this relatively small plot of land is best able to support this heavier eight-story building (plus what looks like a restaurant on the 9th floor). Given its extreme proximity to the Poly, trying to have a completely different themed resort so close would be just plain ugly. Disney is better served trying to take advantage of the Poly's popularity.
FYI, for those who might be unaware, BLT was built on its location because solid bedrock could be reached without having to dig too deeply.
Reportedly, two other locations (circled in red) have been considered in recent years:
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But borings at these sites showed that the bedrock is much deeper, making them prohibitively expensive to construct a heavy building such as the one being proposed.