This document describes the syntax for python annotations. As you can see, they can be any arbitrary python expression, including any random string. Type hinting is just one of the possible applications.

Now, without knowing what was in that string, we cannot know for sure what it was used for in your example, but my guess would be that they are still type hints (as defined in pep 484), enclosed in strings as described in this section of pep 484. For more explanations on that case see this question