The fan bracket screws into the box, using a pair of what are probably #10 screws, which are very long, but I doubt they go any further than the bottom of the bosses in the box.The box only needs to be fan rated if it provides the sole support for the fan. It sounds like the bracket screws into a framing member and not the box.
Some plastic fan boxes also have a metal plate above the box to provide support.
I suspect you have a box that straddles a joist. A pic would confirm this.
Since the fan was mounted this way before, and the other one is mounted same way, I don't think it's going to be a problem, but I think I will go up in the attic and see how the box is secured. Should be easy, since the floorboard was removed or cut to install the box.
EDIT: Just checked the RACO site, and learned of something called "The Insider", a plastic insert for the box when type NM cable is used. This may be what I have. I will have to check from the attic.