The fraction of the genome that is evolutionarily conserved through purifying selection is less than 10%?
I will try to tackle the terminology first

Conserved: That remains identical (or at least very similar) across species.

Purifying selection: Is one of the possible forces that lead to conservation of important sequences of DNA, by removing deleterious alleles.


So, the phrase simply means that the proportion of the genome that is very similar in sequence with the genome of other species (i.e. evolutionarily conserved) is less than 10%.
This is the answer to you question. Then, I just want to add that I share David's view on the abstract.