I think Java used to (until around 2012 or thereabouts) implement String like our Substring type. That is, a string would hold a character buffer combined with an offset and a length. Calling .substring on a string would return a String type (but again, similar to our Substring) with the same buffer, but a different length and offset.
They changed that because the overwhelming number of string manipulations weren't parsers, scanners and other cases where that optimisation mattered. Their String is now like our String.
Swift opted for a middle ground. The Swift project realised that although Java made the right decision when they changed their String, there are still cases where keeping the old "lens" type made sense. So Swift god two distinct types.
It has a different set of tradeoffs. For the most part, the inconvenience of dealing with two distinct types are mitigated by type inference and function overloads, but it sometimes surfaces to the user/programmer. Like it did for the OP.
I still think it is far preferable to having a single String type with Substring semantics.
As the Java team learnt the hard way.