Well, the React PATENTS file seems to indicate that you would lose the patent grant for any patent assertion against Facebook,[1] not just for software patent assertions against Facebook. So, it's theoretically possible that the poster in the parent comment was concerned about a patent on something other than software. Maybe he or she has patented some type of VR headset, for example, and uses React VR for the software that drives it. He or she might reasonably worry that suing Facebook for infringement on the VR headset would result in the loss of patent protections concerning the React VR software.

[1] The language I'm thinking of is the part that says that the license terminates if you initiate "any Patent Assertion: (i) against Facebook or any of its subsidiaries or corporate affiliates, (ii) . . . "