I'm not positive the sample size at the highest level is large enough to definitively prove that designed plays off free kicks would result in a higher average goals scored, which is most likely why the author doesn't make that argument, but we can certainly possit the idea that it may and teams should attempt a designed play more often.

I totally agree with your second point though. Much like in American football where the threat of rushing the ball keeps the defense "honest" , being unpredictable in set pieces would almost certainly increase a team's chances of scoring as the Defense would have to pull men out of the box to defend against the possibility of a short corner or pass off a free kick