I think you might be right. I doubt they have rendered all the tiles though, it just takes up too much space to be worth it... http://wiki.openstreetmap.org/wiki/Tile_Disk_Usage

Looking at that same tile through my proxy (which is backed by the MapQuest map tiles), it is clearly different data.

Here is mapbox, which is also different... http://d.tiles.mapbox.com/v3/examples.map-vyofok3q/15/5242/1...