This is precisely what's happening. The correlation between QB sack percentage and QB yards per carry on rushes (a hypothetical measure of QB athleticism) is 0.66.
When the variance associated with PFF's pass blocking efficiency stat (which by the way correlates with its pass blocking grades at 0.96) is controlled for (i.e., in a partial correlation), the correlation between QB sack percentage and QB yards per carry on rushes is maintained, at 0.67. When the variance associated with the percentage of dropbacks in which there is QB pressure is controlled for, the correlation is 0.58.
In other words, more athletic QBs are having a greater percentage of sacks attributed to them, regardless of the quality of their pass blocking, and regardless of the frequency of the pressure they experience when dropping back to pass.