That's not how it works
The combined strength is used to produce much bigger shockwave, Kaido and Big Mom were surprised even one of the 5 supernovas survived same Kaido who knows Luffy can survive Thunder bagua no problem.
What does exact same method have to do with strength, its a generic way to swing, its the amount of strength put into swing.
This is what Ikokou looks like
And this is what Hakai
You seriously telling me Ikoku looks half as strong as Hakai? Wanker if Hakai was launched inside skull, it can wipe out 20 of those skulls.
Talk about being intellectually dishonest.