That statement is well understood in the Firearms training industry. To remove a gun from a cross draw holster almost always involves sweeping or lasering some part of your body or other directions around you other that the ground and down range towards the target.
Most shooting schools will not allow cross draw or behind the back holsters for training for this reason.
Now cross draw for single action cowboy shooting may be viewed differently, and I am not into cowboy shooting (yet). But for defensive shooting with most defensive pistols (not single action), cross draw is not encouraged.
Cross draw for single action revolvers and and single action semi-autos is the most dangerous since you must be pulling the hammer back as you draw, increasing the likelihood of a drop-hammer discharge.
The only reason cross-draw is allowed at the line is that the firing line is forward of any spectators and participants. The timekeeper stands behind the shooter on the off side. No-one else is in that 90 degree arc. There are berms on 3 sides of the target area itself. Most shooters put the cross-draw holster forward of "normal" carry as well.