I dont think it means that the powers are equal to each other literally. Like i dont think Garp was making 1on1 scenarios in his head. I simply think its meant as a global power representation. The Marine HQ with the admirals being there, plus theoretically having the shichibukai at their disposal, keeps the Yonko in check. The shichibukai and some vice admirals arent exactly standing there on Marine HQ and defending it from an eventual attack at all times, but still, no Yonko is gonna go to a full out war with them (knowing the amount of forces they own, you have to attack with everything you have) and leave themselves exposed to the other Yonko. Likewise, the Marine HQ is not gonna attack Yonko full out, having their hands full with other affairs constantly, and also leaving themselves exposed. Think of Blackbeard. He now has Aokiji as an ally. This alone gives him an advantage over other Yonko, even if there is a high chance Kuzan is gonna backstab. So if you go to war with him, you have to go prepared to deal with Kuzan too. If you go there thinking youll only face the Titanic Captains, its your own fault for getting wiped out
So all Yonko forces vs Marines and shichibukai, i think Yonko would win if you go 1vs1 top to bottom (which is silly as it has no chance of happening), but this system exist simply to keep the powers in check, and the world at peace