I don't think this is true.I believe Florida law prevents them from expanding at BLT because there are less than 40 years remaining. Any construction there must be a new association.
This 40 year restriction is why CCV became its own association and not part of BRV. That was before restrictions.
A new association must have at least 40 years remaining, but I don't think there's a rule about adding new phases to existing resorts.
CCV was a new association probably because Disney thought a shorter lease would be less attractive.