How does it support a 7 speed freewheel when it has a built in freehub?
Basically it doesn't. There are two different versions of the Magic Pie Edge rear wheel, one for freewheels and the other for cassettes.
The cassette version incorporates a threaded freehub which screws into the side casing, whereas the freewheel version has a threaded boss to accept the screw on freewheel, just like the earlier Magic/Smart Pies.
Replacing a worn out freehub
(or replacing the internal bearings) would require a special tool to undo it, although it should be possible to use a 12mm Allen key instead but the stator and axle assembly would need to be completely removed from the opposite side of the hub to allow the Allen key to be inserted.
Alan