Yes, we agree, the rule states there must be pressure from the defense.
Forget about the text in the NFL rulebook definition of the penalty for a moment. My original comment was in regards to a literal dictionary definition of the words 'intentional' and 'grounding'. The most obvious, deliberate (intentional) incomplete pass (grounding) play from scrimmage is the play call that instructs you to just push the ball straight down near your feet, with zero chance of being complete.
On a side note: spiking the ball to stop the clock only came to be in the 1990s. Prior to that the ball had to be thrown past the line of scrimmage and out of bounds to stop the clock. If I recall correctly John Elway was the first player to use this tactic, and did often enough to give Denver an upper hand on the rest of the league for a little while. (I think he and the Broncos were the first to use the forward shuffle pass as well.
I would assume this tweak of the rules was one of many to give the offense a bit of an edge, to create more scoring and to create more dramatic game endings.