The idea is nothing more than speculation.
Whilst it is entirely possible that he made use of an image projection technique (the
camera obscura was known to the ancient Greeks) and traced the projected images I doubt that he used 'photographic techniques' to 'fix' the image.
Think about it.
Natural light coming through a small aperture being focused onto the canvas by a rudimentary lens? The image would be very dim. You could see it to trace it but the sensitivity of the supposed chemicals would be quite low.
Any exposure would take minutes, probably. Possibly a lot longer.
A 10x8 print takes 20 - 30 seconds under an enlarger using modern materials.
Try sitting in one of the poses in his paintings for a minute or two without moving.
And then there is the 'losing' of the technique.
If he used an early form of photography I am sure word would have gotten around and contemporary writers would have made some reference at the very least. The models would have talked. His apprentices would have learned how to do it.
But nothing along those lines has been found.
The simplest and most likely explanation is therefore that he did not use photographic methods but was, instead, a great artist who could draw and paint from life without having to 'cheat'.
And the fact that we have almost no sketches from him does not mean that he didn't make them. It just means that he may not have thought them worth preserving. Bearing in mind that in those days paper was extremely expensive and difficult to get hold of. Paper was regularly re-used when it had served one purpose. I mean, think how much you write in books when you are in school - and how much of it survives past your 30th Birthday.
How many other artists are there who are known to have been prolific yet little of their work survives? Da Vinci left fewer than 30 paintings and most of them are unfinished. And he left no sculptures that can be attributed to him even though he trained for a time as a sculptor.
Why should Caravaggio be any different?
