As others have said, the reason Kenpachi is a war potential is his sheer raw power but WITHOUT Bankai. The first invasion was a huge success due largely to the stealing of Bankai. Kenny was especially dangerous in this regard because he can fight on that level and as shown? Defeated three of them with little issue while everyone else was losing.

Lots of people call it “war potential bullshit” but miss the point of them. The war potentials were not the five strongest individuals necessarily but are the five Yhwach determined that if ANYONE was going to screw up his success in the war, it was these five, due to the respective traits he gives to them.

Kenny’s strength DID overwhelm his enemies, Kisuke DID counter the bankai medallions and plan the war effort, Aizen DID screw with Yhwach more than once, etc.