I can do high speed descents faster on a slick tired mountain bike than on a road bike simply because the added mass bumps up the terminal velocity so it's less of a struggle. You're not proving much beating roadies with a heavier rig.
As an amateur the aero tuck looks like a tempting way to get a marginal gain in speed on the descents. It costs no money and does not demand superhuman fitness. It is easy. But if you get it wrong you get hurt.
My point was that an easier marginal gain is the tyre pressures. So often overlooked, tyre pressures are often gauged by feel for the rider to think their tyres are rock hard for them to actually be half inflated.
If you don't have fitness or an expensive bike then the air in tyres idea is better than the aero tuck for getting some speed.
Regarding your anecdotal MTB speed experience, you have changed quite a few variables - suspension might better suit the road surface, disc brakes might give you quicker braking, the upright bars might help you pick a better line, the smaller wheels might be more aero, the higher bottom bracket might enable you to pedal out of tight turns, the situational awareness from the upright position might give you more confidence on the road and familiarity with the MTB rather than the Sunday special road bike might also be part of the apparent speed gain.
