First and foremost, during the interview it is of no consequence. When they're interviewing you, that's their time to ask the questions and get the feedback from you.
When they give you time to ask questions, you might be able to bring this up as a sidecar conversation. Unless the work is predominantly done on GitHub, it's more of a hobby than anything pertinent to the job you're applying for. It may be useful to get a feel for the kind of person you'd be working with, but I wouldn't dwell or elaborate on it too much.
To the tone of nosiness - if I didn't want anyone to know that I contributed to GitHub, or if I worked at Initech, or if I know Ruby, Java, Python and Kotlin, I personally have the ability to safeguard that. It's fair to look at any information that's made publicly available online, so long as you're not asking pointed and overly intrusive questions about that information.