Polanco has had 2 above average months in a Mariners uniform. He was the only player in the return and was a negative asset for the Mariners last season - the only year he was under contract. As an expiring contract, he really didn't have much value.
Gabby was a slightly above league average hitter in a pitcher friendly league least year, and he significantly improved compared to his time in High-A in the Mariners system in '23. His defense was never a calling card, but a year in which he was dealing with a lower back issue doesn't automatically relegate him to DH unless you're just assuming the worst possible outcome.
Once again, he's played more this year than he did last year. Which sample size means more?
When grading the trade, I think it's fair to compare the received values against each other. The Twins have received about a season of decent, albeit low leverage, relief pitching and a top 100 prospect for a replacement/below replacement season of a middle infielder. Something for nothing is a win in my book.