Some estimates put the number of American citizens pressed into British naval service in the thousands.
Yes, and the number released after verification of their identity was also in the thousands. Per Robcraufurd:
NAM Roger's The Command of the Ocean, summing up the scholarship, arrives at an estimate of c.6,500 sailors [impressed from American ships]. Monroe claimed 6,257 between 1803 and 1812, so the numbers are obviously in the correct ball park.
We can also have a stab at estimating the proportion that were improperly impressed. Press gangs were interested in experienced sailors, not unskilled men they could easily recruit anywhere. Gallatin estimated that of the able-bodied sailors serving aboard American ships engaged in foreign trade, half (9,000) were British- by American definitions, not by British. This compares favourably with statistics from the US Navy at New York in January 1808, where 56% were British by British standards and 45% by American standards.
Taking Gallatin's figure of 50% as an approximation, if Royal Navy ships impressed able-bodied seamen completely at random that 6,500 would have represented 3,250 British and 3,250 wrongly impressed Americans. Because 3,800 men were released on grounds of nationality, there was either a slight bias towards impressing Americans (perhaps reflecting a willingness to accept ordinary seamen or landsmen as well as ABs) or a number of British sailors took advantage of the opportunity to leave Royal Navy service.
In other words, the likelihood is that all or nearly all sailors who were impressed and who were truly American were released after a review process confirmed their nationality.
Unless this was also
de rigeur for slave-taking parties in the South, the comparison falls apart even on that basis.