He was a known drug dealer who was fleeing. I think that is reason enough to restrain him while they see what is going on.
If you are driving and see a cop, do you speed up to get away? If so I’ll bet you get stopped just for acting suspicious. No one likes to see a police car when they are driving. Right away you check your odometer. Guilty people flee.
I would bet if nothing happened to the guy and this went to trial, the judge would toss the knife charge because there wasn't sufficient reason for the initial stop.