Yes an engine fire or an explosive decompression - or indeed, a sudden double engine failure - would certainly take a moment to assimilate !
I certainly took a moment to react the first time I practised spinning all those years ago during my ATPL training ! Even though I was supervised and did it deliberately and knew how to recover, it took a moment in an actual spin to assimilate the forward view and the sensations involved. But to me, being startled means rabbit frozen in the headlights, not knowing what to do next. Experienced airline pilots should hopefully be sufficiently practised in our drills that we are not 'startled' but revert to the appropriate memory drill or whatever.
But during an ILS, if the aircraft suddenly banked over, my first action would be to arrest that behaviour by taking control, and when I said manual control, sorry, I meant flying myself rather than using the ILS coupled mode; not mechanical reversion.
This crew were breathing heavily with their finger frozen on the PTT switch, which suggests they had a very severe control or engine problem, but apparently they didn't and landed normally soon after, without declaring an emergency.
If the 'startle effect' is a thing nowadays, then training needs to be adjusted to allow pilots more Sim practise in experiencing sudden unexpected serious events and performing their memory drills. But an ILS deviation or a failure to capture should not result in anyone breaking sweat.