Not true at all. Swift is a very capable backend language, Apple has open sourced a lot of great libraries to power server software development, and there are projects like Vapor [0] that are used in production.
Ah. You're right, I phrased that ambiguously, sorry.
I meant to point out that there is no apple native cloud solution where you can run swift on apple hardware.
So if your iOS app needs to talk to a backend that you want to develop and host, you need to run that backend on an OS with cloud support, like Linux, some other Unix or windows. But not macOS or some other "Apple cloud" hosting.
For reasons stated above, you might in that case choose Swift.
Ah, gotcha—yes, that is actually a pain point and a strange omission. If you need to run backend code for any reason to support your app, Apple literally offers nothing.
IBM at one point offered Swift "serverless" lambdas/cloud functions, which made me briefly hopeful that Apple could do the same, but that service was deprecated years ago, and Apple has shown no motion there.
[0]: https://vapor.codes