Evacuating into fire/smoke was never suggested - but was there really no possibility of limiting the potential disruption by keeping airside passengers airside, moving some landside passengers to the circular area around the car hire outlets / the baggage claim hall / down the ramp to the coach station, and moving the remaining landside passengers to the area immediately after security (e.g. around duty free) and shopping area immediately after duty free prior to the main departure lounge ? Furthermore, flights which were fully boarded (with no missing pax) could still have departed as normal instead of being cancelled.
That way, the airport would have been able to recover operational capacity more quickly without the need to sweep everywhere airside. The security framework at Stansted is large - contingency planning for this kind of thing must exist or we end up with terrorists seeing even the hint of trouble at an airport as being a way to cause major disruption.