I wouldn't say it's nonsense. I think (for certain people) there is an innate human fear of a singular threat.
For me personally, seeing all the other xenos did take away some of the terror, although they thankfully kept the number low after that. Just like with the first and second films, it felt like the horror had been replaced by action.
I would be much more terrified of being in the water with one shark, than with several, the same with xenomorphs. It's the feeling that comes with the presence of one threat. It can hide easier, it's harder to track, harder to see, harder to hear. It could be anywhere, and the whole situation feels much more intimate. This awful thing that's after you is not just one of many acting as part of a greater whole, it's not some swarm you might wander into the path of. It's mentally dedicated, and it's singularly focused on hunting and killing you. That makes it feel personal, and that's scary.
Plus there's also the chance a survival, a sense of hope that comes with one threat. If I can just do everything right, if I'm really careful, I might just survive. When you're facing a swarm, it all feels hopeless. Hopelessness leads to acceptance and/or indifference, neither of which are conducive to maintaining fear.
But then again, not everybody feels this way. But some indeed do.