“Alternately you can list one or more specific paths (.rb or .rbi files after the tc) to limit checking,”
It might be worth mentioning that the default sorbet/config file generated with Sorbet includes --dir=.
If the user passes specific paths on the command line, those are added to the --dir=. option, so that what will be typechecked is all of . and the provided paths.
Usually, unless a single file does not have any dependencies outside of the standard library, it’s not a good idea to try to typecheck a single file in isolation.
If you really want to do it, you can either pass --no-config, change the sorbet/config file, or cd to a different directory.