If that's not you, then you need to be painfully, painfully explicit about it, and some people still won't believe you because interviewers just lie all the time.
Obviously sharing backend trade secrets would be a big no-no. I've had people say they can't talk about their work due to NDAs, and that's completely fine in my books. We just move on to another topic.
FWIW, even when a candidate mentions having an OSS project, I refrain from looking at the actual code, and especially doing code review. I don't consider an interview setting the appropriate place to be grilling people on why they used a while loop instead of a for loop or whatever, since a) nearly everyone writes "bad" code occasionally and b) often things are done in certain ways for some good reason that isn't really relevant to my goal of evaluating the candidate. At the end of the day, the interviewer's goal IMHO should be to get as many signals they can from the conversation and make a determination about whether the candidate meets/exceeds a minimum threshold of competence
And this is supposed to be one of those bog-standard question that interviewees are always told by job interview guides to be prepared for!
I agree that people that have OSS work to show are much rarer to come by, and FWIW, they tend to do better in interviews IME. My theory is that it's not that the OSS resume bullet point itself sets them apart, but rather that they polished their skills more through the hard work they put into their OSS projects.