If you have a bad cold and your nose clogs up you can also pretty much lose your smell, but then again you also have typical cold symptoms which isn't really common with covid.
Yes, and as Mayo clinic above lists also polips and a fracture, i.e. unsurprisingly if the nose is clogged or its inner lining is altered, you loose your smell, but I would have thought that the sample was excluding these, either because they can be easily diagnosed (polips or fracture) or - as you say - because typical cold (or allergy) symptoms are not usually connected to Covid-19.
All in all the causes are listed under three main categories:
1. Problems with the inner lining of your nose
2. Obstructions of your nasal passages
3. Damage to your brain or nerves
If - as I think - the first two are excluded, the 20% of non-positives but suffereing from anosmia seems very high to me.
