I could clearly see things like access panels and the lines of the undercarriage doors. Not sure I could see those from 1000 feet
You can (even given the average eyesight I have these days). We spend a lot of time 1000' above/below parallel traffic on the Atlantic routes and you'd be surprised what you can see ( but in light of your experience probably not suprised that we sometimes get complaints from passengers about us flying "within touching distance" of another aircraft).
If it's any consolation from personal experience I know how difficult it is to "put a number" on separation distances ( and I speak as someone with a few years in the Air Defence game, decades ago, so I've done my fair share of tanking, close formation, and Vis idents). FWIW we get a numerical display on the flight deck of the altitude difference from nearby aircraft and 1000' can look
very close.
If you were within 150 feet of conflicting traffic the Flight deck TCAS would have been demanding an avoiding manouever for several seconds prior to the "merge" and whilst the manoeuvre is meant to be gentle if the miss distance really was getting down to 100-200 feet you would have been well aware of any avoiding action....I doubt you'd have been reaching for a camera.
Sorry to be sceptical.