From what I read, it was not uncommon for artists of that time used a "camera obscura" to view the images they painted.
That's what I've been reading too. The ability of a pinhole to form an image in a dark room had been noted by Aristotle as well as by Chinese and Korean scholars, and an Arab physicist fully described the camera obscura by around 1000 AD. Leonardo had written about it a century before Caravaggio's time.
What's controversial about Hockney's (and his collaborator Falco's) hypothesis is whether or not artists of that time period actually used it. Hockney and Falco claim to have found areas of specific paintings that show evidence that they did; other artistic and optical experts say they don't show that.
That controversy isn't new, though, so what's different about this article is the researcher's claim that Caravaggio used the crushed-firefly goop to
record the images. I think the article slightly overstates the researcher's contention, though, in saying that Caravaggio was able to "fix" those images so they would be permanent.
The way I interpreted the researcher's remarks was that Caravaggio would use the firefly goop to record the image
temporarily, then use substances such as white lead and mercury to mark the outlines before the glow faded.
What I don't get, though, is why Caravaggio would have had to bother. After all, there were plenty of other ways to get realistic-looking perspective relationships that didn't involve crushed fireflies, lead, and so forth. The researcher admits that there's no way to tell whether or not Caravaggio did this except by chemical analysis of the paintings, so until some Caravaggio owner is willing to step up and try it, it's just an interesting but unprovable assertion.