No mobility? False:
Darius' basic attacks and damaging abilities cause enemies to bleed for 12 / 15 / 18 / 21 / 24 / 27 / 30 / 33 / 36 (+0.4 per bonus attack damage) magic damage over 5 seconds, stacking up to 5 times.
Darius gains 5% movement speed for each bleeding enemy champion.
Crippling Strike: Slows enemy by 20..25..30..35..40%
Apprehend: Darius pulls all enemies in front of him (holy **** this thing's range is giant)
All of those abilities function as gap closers or increasing Darius' mobility. I don't really see the argument valid for his lack of mobility unless you include the lack of a dash. Darius can pull carries out of their positioning, or just proceed to execute dunkomania upon the rest of the team. A good Darius, like a good Garen, will realize the exact moment at which they should proceed to dunk which will likely result in a dunkomania.
also, you could add to your description, that Decimate applies his Passive (and has a huge range), effectively gaining 5% MS and can even be activated DURING movement