I dont think its enough to just look at a guys physical profile to characterize them by position. 6'9" 228 was LeBron at parts of his career, Thad Young, and Tmac. One of those guys was a guard half his career, the other a big man. It is a question of where Diakite likes to operate on the floor and how we can best utilize him defensively.
The guy is a plus rebounder and weakside shot blocker. Sending him to the perimeter to chase around the Kawhi's and Klay's of the world takes away what is his biggest strength. How is he running around screens? How is he shading a shifty ballhandler 1v1? Unless he can do all of that he is a small ball big in the vein of PJ Tuck or Patrick Williams rather than a perimeter defensive wing like Danny Green or Reggie Bullock.
On offense, he can space but you can do that from 1-5 so no big deal as he would never be more than the 4th option. IMO, starting 3 is not possible for Diakite. There might be situational lineups where he is the nominal 3 on defense, maybe something like a Diakite/Love/Allen frontcourt, but that should be a rarely used tool in the toolbag.
Lauri at the 3 worked because of his offensive and defensive fit. Despite his height and length, he was soft on the interior. Not a bigman level rebounder or a high level rim protector. His long arms made it hard for perimeter players to throw lazy passes and he had surprisingly solid balance for his size, which let him stick with larger perimeter guys. On offense, he is a bonafide sniper (better than Diakite by a notable margin) and had enough juice off the dribble to be a 3rd-4th option as a starter. Even if you resized him to have Diakite's height and weight, he would've been a 3 and vice versa for Diakite going to Lauri's proportions. Just totally different skillsets between the two guys.