If Facebook has patents covering React, these probably also cover Preact, so you'd actually be in a worse position: Potentially violating patents and not having any sort of grand.
If anyone had a patent on virtual dom / diffing, then they could sue anyone else that used an implementation that was implemented in the same way as the implementation in the patent. That doesn't then mean that they "own" those implementations, only that they can use the courts to stop others using them or pay to licence their patent.