Why would React be impacted? React is open source no? Anybody can contribute to React.
Maintainers and major contributors are from Facebook. You can't "just fork" the monstrosity that React is and magically keep it moving at the same pace.
For the second, the question "Why would React be impacted by layoffs if it's open source", the answer is obviously: It would be impacted if people who were getting paid to work on it have been laid off. Seems straightforward. Bigger impact depending on number and roles of such people.