> You do seem to get more | shaped pieces when you leave those spaces open on the board.

I don't think this is correct, nor that it can be evinced from the article. What it does say is that the sequences that led them to achieve their target show a higher incidence of I shapes. This is because all the ones that show less I shapes have been "pruned away" by the cost function, which favors I shapes.

This has some relationship with the anthropic principle: isn't it strange that, of all the possible universes, we ended up in the one that seems fine-tuned exactly for life as we know it?

Nope, piece selection in the unmodified NEStris uses only the player input to generate pieces and it also starts from the same seed, so frame-perfect inputs after loading the game will result in the exact same pieces. The first perfect level 18 run (only tetrises to score) was dismissed by community because it was made by replicating a TAS run with a suitable piece sequence.

Their strategy optimized for the I pieces, so it should be no surprise that winning runs were ones that had a higher incidence of those pieces. Runs with fewer would not have survived under that strategy.