your answer makes it clear that you must not have built anything yourself of substantial size or complexity. if you worked on something for 10-15 years chances are you cannot answer some random specific question about that project because you haven’t worked on that particular part of the code base recently. so that does not mean you are lying about having built the thing.

my point is that assuming everyone is lying, biases you to the extent you want the outcome to be that the person is lying such that you can pat yourself on the back for a good job done rather than coming to the awkward conclusion you were wrong. and that is a real problem in interviewing because it means you discard top candidates.