Maybe I don’t remember this in details from the movie, but doesn’t this exactly make sense. The son wipes the knife free of prints, then runs, and this is in time for the old man to see him.
Maybe she wore glasses normally, maybe she had just worn sunglasses. Maybe she was reading in bed. It’s at least a little bit of a leap from marks on her nose to concluding that her sworn testimony is certainly false.
So, you’re saying the son take the time to wipe fingerprints off the knife, dashes out of the apartment, then goes back hours later, supposedly to get the knife?? How does that make sense?
It’s enough to cast reasonable doubt on her testimony, especially (as stated earlier) she would have only glimpsed the murderer through the moving windows of an elevated train, at night. Even if the cars were empty (no guarantee there) those are piss-poor conditions.