How was the toilet attached to the floor or this flange?
It appears that the flange has been sealed with glue or epoxy. It is almost certainly threaded into the tank, but sometimes folks will do something silly like glue them in when they are stripped or the tank fitting cracks. (Usually from overtightening.)
The flange shown would use a foam seal as others have indicated, but it doesn't appear to have any holes for the bolts. I'm assuming that the toilet clamps down to it.
Anyway, let us know how toilet was attached and whether you are reusing toilet or not.