Could data from a test set 'leak' into predictor during PCA?
The author is right.  You are wrong.  There's no such thing as "only metadata leak" or "can't leak really".  It's like saying "not pregnant really" -- either you're pregnant, or you're not.  Same here -- either data can leak, or it can't.  In this case, data can leak.  Maybe only partial data, but it's hard to know just how partial and just how bad the impact of that might be.  It might be very bad, or it might not; but since you can't know, testing with a methodology where you know the results might be meaningless isn't a good idea.