It's very strange that you think that a GM constructs the "team of coaches." Generally, coaches are responsible for their own staffs -- it's usually part of the recruiting/interview process to discuss the staff that you would and can bring with you to create your team.
You could easily argue that the WORST part of Keim's job is his ability to build coaching staffs.
I'd say that drafting is the MOST important part (especially post the last CBA that capped rookie deals) because it creates so much flexibility along the rest of your roster.
Here we are in mid-April and we need at least 4-6 starting players because we failed to draft or retain home-grown talent; we'll definitionally have to pay more for veterans at those positions, and have them on shorter-term contracts.
That means will have to play the lottery again next year and hope that we don't roll snake eyes on another position that we need to sign a veteran at because we couldn't retain someone.
I'd argue that Keim's record is better because Bruce Arians excels at getting the most out of a roster from the W-L perspective, and it had little to do with the rosters that Keim placed in front of Arians. The evidence for this is how Arians managed to go 7-9 with a Bucs roster with Jameis Winston passing 30 and 30 TDs and INTs plus the 29th-ranked scoring defense in the NFL. Add a respectable QB to that roster and Arians is lifting the Lombardi trophy.
With another head coach, those last two Arians seasons probably look more like the last two Whisenhunt seasons and the one Wilks season, and you have extremely similar records for both GMs.
I feel like this take aligns exactly with the logic I posted above.
Keim has had 3 HCs. Two of them he hired all the coaches.