Once the slides are deployed they get an indication in the cockpit. I reckon they knew whats going on, only that they didn make the decision for it.
As a pilot if you see the slides are deployed what would you do? would you shut the engines and go out of the cockpit to see what the ..... is going on?
There are two different perspective here.
1.You have the cockpit, confined, no burning smell or very little, going trough checklists.
2.You also have the CC with 200 pax on board looking at them and a strong burning smell they have no ideea where is coming from. CC is thinking if its safe to stay on board or get out, they wait for some info that doesnt come.
Purser tried to get more info, couldnt, so got everyone out.
Cant blame any of the parts involved but IMO FD should have had a quicker reaction to either communicate with the CC/pax or initate the precautionary disembarkation.