Open Source has nothing to do whether data involved in the program/service is available, it refers to the source code. This source code is the recipe of the program, it is written in a programming language. When a project is open source people who make modifications to that code must publish the modifications with the same licence(generally speaking), thus ensuring a mutual knowledge exchange.

As for the data in Lemmy, yes you can see who upvoted posts and comments but it is not available in the interface. There is a website called Lemvotes or something that does this. To be clear, a project can be open source and not expose data of their users, for example the Matrix protocol.