The #if swift(x) conditional compares against the language version, which is different from the version of the compiler that you're compiling with. It is possible for the Swift 5.7 compiler to compile in Swift 4 language version mode by passing the flag -swift-version 4 when you invoke the compiler. When you do this, the compiler(>=5.7) check will pass, but the swift(>=5.7) check will fail.
It's somewhat curious to me that the swift check accepts arguments that aren't valid language version. The command line will reject any argument to -swift-version that isn't one of 4, 4.2, or 5, so swift(>=5.7) basically acts as shorthand for languageVersion(>=5) && compiler(>=5.7). Again, though, perhaps too late to change this.