F-15s took off from McChord Airbase 5-6 miles south of Tacoma.
Sea-Tac is midway between Seattle & Tacoma perhaps 20 miles north of McChord.
McChord airbase is about 4 or 5 miles due east from Ketron Island.
The alert fighters are based in the small hangars near the south end of the runway.
The fighters would be airborne within a minute or two of the order to scramble being given.
For scale the McChord runway is 10000ft long (approx 2 miles).
Yes. Think of it as an air display or film stunt. you see the plane cavorting and see/hear the fighters scrambling. The fighters are with the plane almost as soon as they get their gear in the wells.
In the movies you see a plane crash but in reality it dives behind some trees or a hill & at the right moment a large flash/bang device is detonated.
In the distraction of the explosion & the fighters the Q400 slips away at very low level.
Everybody sees what they expect to see.
It is smoke and mirrors and sleight of hand just like a stage magician.
That is local law enforcement circling the crash site.