I'm not debating that (hence "other than exposition"). I was just countering your point that we actually saw what a tactical genius he was; he was anything but in that episode of TOS. Even a friggin' psych major was able to thwart his plans.
Garth may have been a tactical genius in his heyday, but by the time of "Whom Gods Destroy" he was a completely insane megalomaniac hellbent on ruling the Galaxy.
If Garth appears in Discovery, it would be in a transitional state, probably right after his galactic fame as a tactician and before his complete descent into madness.