I'm not so sure I buy this. I don't recall these "mortar kicks" being used back before the kickoff was moved to the 30 in 1994. If there was some advantage to this, wouldn't kickers have been doing this pre-1994 instead of just booting the ball into the endzone?
(note- I have the argument in my head, I may botch it putting it down in words)
Kickers also didn't have "the leg" that back then that a lot of kickers have today. The whole reason why the kickoff was moved back to the 30 yard line in 1994 was
because there were too many touchbacks. If they were to "mortar" a kick back then, the returner would likely field the ball at the 5-10 yard line and easily get past the 20 - statistical advantage to kick a line drive and put it into/through the end zone. Move the ball back 5 yards - if you kick a line drive kick into the end zone your coverage has no chance to get down the field, so even a kick that goes 5 yards deep into the end zone has a chance to be returned to the 25-30.
With stronger legs, a kicker can "mortar" a kick and get it to fall between the 5 yard line and the goal line - much harder to advance it past the 20 yard line, especially since the wedge has been outlawed.