Without even mentioning the inadequate sample size, the trouble with attempting to isolate just match prep sessions or just set piece sessions is that it ignores so many factors that are supposedly linked together.
It's a mistake to run test with both teams at 100% tactical familiarity.
It's a mistake to ignore whether players are or are not on PRD and/or additional focus training.
It has been widely discussed that the match bumps do not stack, so running the set piece test with a bunch of attacking corner or attacking free kick sessions, according to what we supposedly know, shouldn't result in massive increases in goals from corners/FKs. What is worth investigating is whether this many sessions has bumped the attributes claimed by the card.
On top of the tactical familiarity flaw, the test does not account for morale, training happiness over the course of a season.
A better test of match prep and set piece sessions would run for multiple seasons with Team A and Team B set for identical training, excepting the match prep/set piece sessions, and equal, but lower than 100%, tactical familiarity at the beginning of each season. After watching several of the videos, I'm not prepared to completely throw the baby out with the bath water regarding match prep or set piece sessions.
What is significantly more concerning is the testing that suggests the training cards do not actually boost the attributes they claim to boost. I believe there may be something to this, as, anecdotally, I constantly see instances of targeted attributes barely increasing across a season or two in players that are in the 18-23 range. If I have wingbacks set to WB-At PRD with additional focus training set to crossing and I routinely run attacking overlap and play from the back sessions, it's absolutely absurd that I shouldn't see noticeable improvements in crossing, flair, passing, vision in at least some of those players and in most they barely move.