I think it's single in the sense that only a single failure in any of those points would render the entire project a failure, not that there's only one point that can possibly fail.
Contrast, say, a single-engine jet plane with a twin-engine jet plane that can still make it to the airport safely with the remaining engine, should one engine fail mid-flight.
Because a point of failure could have multiple redundancies, and in this case they did not, every one of those points was implemented in a non-redundant way.
Most failure points on Webb have redundancies. So their failure doesn't brick the telescope, both the part and its redundancy have to fail before Webb is bricked. The single points brick Webb with only a single failure.
It's a point where a single failure causes system failure, but that's a lot more words. These things tend to get whittled down to "terms of art", the meaning of which is understood by those trained in the art. It's perhaps best to just call it an SPOF and then people will hopefully google it to find out what that means.
