Why was there the delay? Is it that long to shut down the engines, perhaps that long to spool down?
The need to evacuate _seems_ clear. Keeping on board with flames under a wing/engine seems like a not-so-good plan, or at least one that seems likely to lead to panic among close-by passengers.
I do recall the debate on a similar incident on a wide-body somewheres in asia recently, i.e. that maybe fuel spill spreads under airplane means neither side is safe. So need to make sure, but surely that has to be based on what can be easily seen -- you can't wait 2 minutes for fire trucks to get a better view. As pointed out: Airtours or PW (or was it CP) in Calgary.
Instead of telling people to sit down, a not-so-convincing command in that situation, maybe tell them to get ready and remind them to leave everything behind, can get it later. The sit command, to me, leads to mistrust -- and as someone pointed out: fetching suitcases from overheads.