Why not? It's a single thing, that if it fails, causes your app/website/whatever to fail.

Because it isn't a single thing, it is a redundant system. Redundant systems can also fail, that doesn't make it a _single_ point of failure.

You can add another system in parallel as the vendor of the product suggests, or you can improve the resilience in the redundant system.

To make a hyperbole: my galera cluster is failing, its a single point of failure, so I setup a cockroach cluster in parallel.

In a way, it is right, as there are failure modes specific to the individual systems, but I think, it is incorrect, to label that a SpoF.

That is a fair point; SPoF depends on what level you're looking at. A RAID array removes the SPoF that is a single disk, but still leaves a SPoF in the RAID controller or CPU or power supply; a ceph cluster can withstand the loss of a whole rack but could still fall to certain software bugs. Likewise, "cloud" companies are internally redundant, right up to the point where they aren't. It depends on how you scope the question.