Based on what exactly? It was basically just Hakoku Sovereignty, but the Kaido and Big Mom version. Which is literally just Ikoku Sovereignty being done by two people. So it was pretty much BM's signature basic technique (which, bear in mind, a heavily damaged and fatigued Kid took directly to the face and shook off), with Kaido doing the same thing. It was really strong, but.....strongest?
Based on the fact that combos are far stronger than anything individuals can accomplish, this is not a mere 2x multiplier.
Dorry and Brogy can't achieve anything even close to their combo, individually.
Luffy and Zoro cant achieve anything even close to their combo vs Aqua Laguna, individually.
Monster trio cant achieve anything even close to their combo vs Pacifista, individually.
BM's Ikoku is dog shit that Luffy easily dodged.
Hakai was not dodgeable, not even close.
Ikoku is maybe 1/100th part of Hakai and thats a big maybe.
No, Kidd did not take Ikoku to the face, get your shits together.
Whoever thinks that Imu's mere swing of an axe is anywhere close to Hakai is utterly clueless and shouldn't powerscale ever again.