I think the interpretation intended there is that if you hand a representation of a file to a tool, it needs to be immutable.

The representation likely is full of string refs and pointers to the actual data in memory to prevent egregious copying, and the caller of the api should not be able to modify the data structure they receive and accidentally modify the actual file unless they do so through an api intended for it.

Nothing to do with copyright, more that swift-format or your lsp integration should never accidentally modify a file without user initiation.