They use HEAT warheads for anti armor duties. Speed of the projectile is near meaningless for that kind of warhead. We've seen hundreds of footage over the years, if an AFV is hit by a MAM-L it's either dead or mission killed.
Using UMTAS on drones in place of MAM-L opens up possibilities for greater flight envelopes for munition firing, removes some of the altitude requirements to achieve the reported range and keeps the firing platform further away from the target, lowering risk of it being shot down. It being laser guided still limits the possible range, AF500's laser designator have what, 20km range? That'd be similar to MX-15Ds.