They can 'come after you' iff you have accused them of that, and you are using their software in such a way that a parent of theirs is violated.

It's pretty straightforward:

- Facebook has released some open source software. In the case of React, it's under the BSD license.

- You can use this software however you want, in accordance with the license.

- In addition to granting you a license to use the software, Facebook has granted you a license to any patents they own that cover the software.

- There is no enumeration or claim that any patents do cover the software.

- Your license to use any hypothetical patent that does cover the software will be revoked if you make a claim—legal or otherwise—that Facebook has infringed any patent.

- At that point, you will no longer have a license to use any patent that covers the software.

- If you are subsequently using the software in a way that infringes one of their patents, Facebook would legitimately be able to claim patent infringement.