The risk behind Preact isn't the API. Remember, this is a discussion about patents, so what matters is fundamental concepts and algorithms.
The most likely thing for Facebook to patent is the concept of a virtual DOM that's diffed to apply updates to the real DOM. IF they have such a patent (and apparently they don't), then any library that has a vdom implementation infringes, including Preact.
Of course, if they DON'T have such a patent (which seems to be the case), then Preact is safe, but so is React. :)
The most likely thing for Facebook to patent is the concept of a virtual DOM that's diffed to apply updates to the real DOM. IF they have such a patent (and apparently they don't), then any library that has a vdom implementation infringes, including Preact.
Of course, if they DON'T have such a patent (which seems to be the case), then Preact is safe, but so is React. :)