Agreed on both accounts. I’ve done some cursory searches for patents covering Virtual DOM and was surprised to find very little on the subject. I believe this is because a diff algorithm and DOM interactions are both not inventions (the former is a group of known common algorithms, the latter an open specification) — simply applying one to the other does not satisfy the criteria of being a sufficiently non-obvious invention.
Your “devil is in the details” is the perfect description of this though — I do believe Facebook has at least one patent covering some of React’s more inventive internals. Concretely, they have a patent covering a technique for culling out-of-view nodes in a (virtual) DOM structure that could certainly be apply to React’s recent “Fiber” rewrite.
I think it’s worth noting that, even in real cases like the above patent, Preact would not infringe — where React is a well-designed combination of algorithms that accomplish a goal, Preact is simply a “glue” layer between JSX (an open standard created by Facebook) and the DOM.
I think I could summarize my take on the matter thusly: parts of React are certainly patentable —some that come to mind would be abstract renderers, Fiber’s stack implementation, and React Native. Replicating these things in another library is potentially dangerous, and I would recommend developers avoid doing so. However, none of Preact is patentable — it’s far too obvious. Preact simply connects two open specifications in the most direct manner possible. Preact can be thought of as a coupling mechanism, like an electrical outlet and plug.