The modified dates on Place Properties refer to the time those key/value pairs were last updated in the database. Because those don’t change very often i.e. the URL is always the same for a given camera, the modified date will not change unless the database is rebuilt (roughly weekly, sometimes more often) whereas the image on the other end of that URL can, and does, change constantly. But the URL itself doesn’t change, so therefore the modified date doesn’t change.
If you look at the modified date of a property that does change often, e.g. NbBikes on a BikePoint, you’ll see that it does update along with the value in the key/value pair whenever the number of bikes at the docking station changes.
In other words, the modified dates are an artefact of the database’s internal change tracking and can be ignored.
(Disclaimer: I no longer work with TfL but still lurk around in case I can help - this one triggered a memory!)