Treinen's 1.93 ERA in 46.2IP last season is also good, but If you're holding his recent performances against him then you can't just ignore Kopech's.
I disagree that the stuff is the same. Fastball is slower/straighter/wilder with a career worst K% and he doesn't throw a slider anymore. Career high pitches per batter/inning because his misses are too uncompetitive to get chase. Between OKC and LA, he has walked 33/122 (27%) of batters faced this season.
And that includes his stats before the knee surgery, he's been even worse since then. 9 walks (and a HBP) vs 5Ks across ~32 batters in 5.1 rehab innings in AAA. 9 walks vs 4Ks across ~24 batters in 4 MLB innings.
A guy with twice as many walks as Ks, allowing a free pass to every 3rd batter and 3 baserunners per inning, just does not scream postseason leverage arm to me.