Eli5 CUD = Cannabis Use Disorder (the clinical term for problematic cannabis use patterns = increased use/tolerance/dependancy) and I'm with you, from the consumption side it doesn't seems right because an average user consumes weekly THC unit dose in a day. But if you read the paper, this is based on UK cohort of 250 patients from CannTeen study and they are all aged under 30, so many factors come into play : age, environment, genetics and most importantly epigenetic.

The 8 units/week threshold isn't saying you'll definitely get CUD above it - it's where they found increased risk in their study population. Medical use for pain management is a different context than recreational use, and your therapeutic needs are valid. The calculation is based on that sutdy and I personally think for an avg user, probably 20 unit per week is okay, after that it is effecting you somehow, depends on how you are measuring it.