If Sorbet knows the type of the receiver of the method call, it will suggest completion results, regardless of whether any of the suggestions themselves have sigs, or whether you're inside a method with a signature.
But without any sigs, there aren't going to necessarily be as many opportunities for Sorbet to know the type of the receiver as there would be with sigs.
Here's an example
https://sorbet.run/#%23%20typed%3A%20true%0A%0Aclass%20A%0A%...