Both over the cap and spotrac dont differenciate between CB and slot CB, to be honest, offenses run with 3+ WRs on more than 60% of their snaps these days so slot corner has become a more important position over the years.
This is not the perfect way to account for the lack of specific data, but if we include 3 CBs per team(top 90 NFL CBs), they average 5.1M/per, which is still more than your average starting FS...
At this point we'll just agree to disagree, I believe an NFL defense as alot of ways to use Minkah outside of the FS position...Thats pretty much my stance on it...