"How do you differentiate between the dreams being detailed and the dreamers simply being convinced of the dreams being detailed?"

A good question. I've developed the same test that the "rationalists" sometimes use to tell if you have an opinion. If you can remember something in one way, and it seems to fit, but you can remember something in another and it equally seems to fit, then you don't really remember the detail of the two ways it seems to fit, you just have a vague idea.

I can draw you a map of my (real life) office. It will be fairly accurate within what I can see, and I can put down specific names. If you swapped two names, I could figure out that does not corresponding to my memory of the office. I remember when we went out to a team lunch two months ago, and I remember the specific order we sat in the booth, and many details about the specific booth we sat in. And to be honest, I would rate my memory as below average.

By contrast, I call to mind a dream right now in which I encountered a crowd of creatures on a hill. I definitely remember it was a "crowd". But... were there a few of them? Yeah... I suppose that fits. Or were there hundreds of them? Yeah... I suppose that fits. I remember they were about 1 foot high and basically the critter from Aliens... but did they have claws or hand-like things? Either fits my memory. Did they have long tails? I think so, yeah. Were they articulated or solid? Yeah... either way, really. I remember them as a "crowd of creatures like the aliens", but when I really drill down... I don't actually have that many details. Were I an artist, I could draw you a picture that fits my dream fairly well... but I could then turn around and draw a picture that in the waking world and light of day would be fairly substantially different, but that my brain would say matches the dream just as well.

I've read things too... but... what color were they? Usually I don't have an answer to that. (I think the common idea that "people dream in black and white" is inaccurate... people dream in not color. They encounter concepts like "words written on a sign" and they just plain don't have color at all. I occasionally do remember explicit colors, but it's an exception.) What was the font? Was it all uppercase? Yeah... maybe. Was it spread across multiple lines? I dunno, I can remember it either way pretty equally fluidly.

Don't get too caught up on these specific examples; for instance I can call to mind a specific instance where I would say concretely this text was on two lines, specifically. I'm not saying these details never appear. What I am saying is that for most people, I suspect if they examined their own dreams like this they'd discover it has less concrete content than they thought.

I would also suspect that there are the exceptional people out there who do dream highly specific dreams every night. But I bet they're the exception. Hard to know. Given the evidence that the act of recalling memories involves destructively reading them and then rewriting them back out, I would also suspect there's another group of people who actually have vague dreams and turn them concrete in the very act of remembering them, but I won't be able to prove or disprove that hypothesis pretty much until we have full brain simulation, so take that with the appropriate levels of salt.

(This is also useful for real life memories. I chose concrete memories I'm confident about like my office layout. By contrast, something like "the wall color or pattern of the hotel I visited a couple of months ago"... well... I know it's in the "boring beige" spectrum but beyond that I can mentally decorate it in any number of specific ways without it triggering any conflicts with my memories, so, I don't remember that. By contrast, I do remember the layout reasonably well. But I don't remember the counter colors, or even whether they were formica or granite.)