Logic would indicate that stat reads as too good to be true.
Transfermarkt lists the following ppm values for our last 4 managers you mention.
Neil Harris 1.44
Russell Slade 1.43
Neil Warnock 1.43
Mick McCarthy 1.39
So whilst they "all had 1.43 ppm" stat is incorrect, it's clear there's a narrow range in place - if the stat can be believed.
Logic would indicate that the figure is artificially affected by the fact the ppm calculation includes all matches managed, which includes cup games. This means matches, where in effect the opponents aren't always regular opponents - possibly lower league, possibly higher league, means that team changes occur that wouldn't necessarily occur in league games.
Logic also indicates that the league played in also affects the figure. Premiership? Always going to be up against it, looking to stay up. Championship? Top ten side, there or thereabouts.
Logic therefore concludes that the assertion basically a top 10 side at this level is probably accurate, for many factors other than purely manager ppm.