While it's true that HT(And even SS themselves), have moved on with leaps and bounds making two-piece boots that have articulation at the ankle, the boots that were worn in the movie were Frank Thomas Titan 101 motorbike boots, and those have hardly any lateral articulation in real life. It has no bearing on a 1/6 figure, but those kinds of boots are supposed to be fairly rigid in case you fall off the bike to protect your ankle. They have a little up and down movement so you can use the back brake and shift through the gears, but won't move much side to side.
It comes back to the old conundrum of whether you want it accurate to the actual boot, or want more posability. Half will want ultimate accuracy, and half will want a bit of ankle motion. Sideshow probably went with the easiest answer. A solid boot, like real life.
For someone who wants them a bit more movement, i expect if you bought a loose pair of boots and a TDKR pair of bootleg Batman boots(Which have separate shoes and boot sleeves), you could cut up the maul boots and slide them over the batman boot sleeves to get movement.
Seems a lot of work unless you are going to be posing him doing the splits in your detolf, lol.