As the name suggests, a microatheroma is a small atheroma. An artheroma is what's commonly known as a plaque. It's a mass on the interior wall of arteries:
atheroma [ath″er-o´mah] an abnormal mass of fatty or lipid material
with a fibrous covering, existing as a discrete, raised plaque
within the ...
I think this is probably due to selection bias, but I cannot find a source from someone in the field to reference.
Instead, to support this conclusion, note that in the review you reference, the people with near-occlusions overall have fewer negative outcomes than those with lesser occlusions.
People who are not already dead while having such a blockage ...