I think it makes perfect sense. The manga is explaining to us the difference between Big Mom's durability and recovery compared to Kaido's and what is needed to defeat them.
First, with any type of battle the more damage an opponent receives and the longer the battle goes on the more they're going to become wore down.
Big mom's durability represent the rock while Kid and Law's AP represent the drip of water. Their AP (drip of water) may not be as strong Big Mom's durability (rock) but if they continue to attack Big Mom they will defeat her durability and cause her significant damage. Just like a drip of water can defeat a rock over time they can do the same with Big Mom.
Big mom has a durable body because of her giant blood. There's nothing supernatural about it and it can be defeated. You don't need specific type of attacks or the strongest AP in the manga. Of course, some attacks work better than the others and the greater AP the easier and faster it will be to defeat her durability.
Kaido is different. His durability and recovery comes from his devil fruit. It's supernatural and and it makes them nearly impossible to hurt and up to this time unkillable. Powerful Ryu or attacks that cause internal damage plus some of the highest/strongest AP with COA/ACoC is needed to cause him significant damage.
Kaido can't be defeated by just hitting him with powerful attacks over and over again. That won't work. Even if you can damage him the damage would be very insignificant and he's going to quickly recover. That's the reason Luffy earlier rooftop attacks were insignificant and the Scabbards combination attack couldn't reopen his old wound.
This can't be it for a simple reason
The example signifie the number of attacks, a drop of Water won't do anything but a huge number of these drops will eventually do the trick
Now who took a great number of attacks? It's Kaido hence the drop of waters will affect him eventually
Kidd is partly right since Kaido is weakened right now
but BM only took 4 or 5 attacks at best, that's not drop of waters affecting her slowly, the damage is being done obviously with a low number of attacks
and with Kidd saying he won't let BM go to the roof and talking about a different purpose, Oda intentions are obvious