source: https://nextgenstats.nfl.com/stats/receiving#average-separation
This is correct. According to NFL Next Gen stats, Samuel had an average separation of 3.3 yards while Diggs was only 2.8 last season. Furthermore, it's something consistently that Diggs' separation rate is between 2.5~3.0 yards in last five seasons while Samuel is consistently half yard or more higher than Diggs in separation.
And Yes, Davis' separation rate is in the same range as Diggs and lower than Samuel in last 5 seasons.