The Big Sur SDK (evidently) uses features from Swift 5.3. Since Swift 5.2 is forward-compatible with Swift 5.3, any existing apps will should work when compiled as Swift 5.3. However, a Swift 5.2 compiler won't be able to use the Big Sur SDK; thus far, Apple hasn't put effort into making sure a new SDK compiles with an older version of the Swift compiler.* The compiler doesn't (yet?) have features for that either, though it gets discussed every now and then.
* Historically, Apple has put a little more effort into making sure their Objective-C code compiles with older versions of Clang…but only while it's not an undue burden. That is something that's easier with a preprocessor, though.