When I was a kid in school and even when I was in college, we were taught that oil and coal came fromt he same source, biomass conversion. The fossilized leaves, etc. found in coal were shown as proof positive. Interesting how such myths take hold and are so difficult to shake off.
Well... isn't that pretty much true?
Coal-in it's various forms, oil-in it's various forms, come from biomass. Although that may not be the only 'source' for coal/oil.
Diamonds were supposed to be formed by tremendous heat and pressure deep within the Earth. Now we think it may have been produced by meteorite impacts. After all, many diamonds were found on the surface. Maybe they were formed also, by both.
Many things that we 'were' taught , we now know are not true. Many things we know 'now', are not true either.
At least when we were young, teachers TRIED to tell us the 'accepted' truth. Nowadays, it seems most 'teachers' are only concerned with paychecks and politics.
You are (more or less) correct.
Biomass conversion depends upon depth, temperature and possibly other factors.
Scientists try to define it but even they disagree on most aspects.
Therefor - the confusion.
Guess only God knows for sure.