40% isnt arbitrary, as kayle pointed out, but it is being misused
40% is the odds of getting at least ONE sableye in your first 7 cards, but that isnt the same as STARTING with sableye, as you may mulligan and get an opportunity to draw sableye in the mulligan.
he could/should say you start with sableye at least 40%-- that would be correct and supported wholly by probability