I never said May is the "best they can do." But he is still better than most, if not all of their current rotation.
You summed it up pretty good. High(er) strikeout rate, low walk rate is a damn good place to start for a starting pitcher profile. May is the only SP on staff with an above league avg K/9. Has also limited homers decently.
May's FIP is 3.16, over half a run under league average. Nobody else on the Twins is under 4. His xFIP is 3.86. That's about as promising as it gets on their current major league staff.
His hits allowed numbers were getting better as the year progressed, which would be expected for a guy coming into the season with fewer than 10 starts.
Side note, if you're worried about hits allowed, why would you say Kyle Gibson (pitch to contact disciple) is "good," yet Trevor May is not promising? May H/9 through 20 starts is pretty similar KG's, and is still improving (as did Gibson). Also, Gibson FIP is 4.10, xFIP 3.81.