This is, at best, a poor interpretation of their own rules. Passive checks are nice because they avoid repetitive rolling that players might otherwise ask for, particularly if you require an action declaration. The passive roll sets the DM up to bypass that need for something that should be an ongoing process - observing the environment around the PCs.
The page talking about it is: Does passive perception supersede active perception?
If they would have noticed.on a passive roll, should they have been told they spotted it before they even got to request a roll (assuming they were progressing in there usual fashion, and that includes trying to keep an eye on the surroundings?).