I think it's just command. If you look at RP samples from the last few years with 10+ IP and then sort by K% and eyeball for RPs who have had TONS of Ks but posted poor ERA and FIPs (read: got hammered while getting tons of Ks).... you don't get many but you see guys like Ray Black, Zac Rosscup, this s***** version of Craig Kimbrel...
And his Stuff+ numbers support this
104.8 stuff+ and 94.3 command+
Compares to guys like Jordan Holloway, Matt Barnes, Jared Solomon, Gregory Soto, Yoan Lopez, Tyler Matzek, Andrew Wantz, Michael Fulmer, Jovani Moran, Corey Knebel
There is no longer much reason to believe he can be a good starting pitcher. But there remains some reasons to believe he could even out into a pretty good reliever. Some of those reliever names are actually solid enough to help a good team.