Momonosuke means the strongest in the world
That is your head canon. Neither Oden was the strongest in the world and nor will Momo ever be
he's oden's son and he's a dragon. Making the theory valid
That is really lame excuse. Oden knew haki but he travelled with WB and Roger and kept honing his skills and fought Kaido 20 years ago with everything he had at the end of his road. Kaido since then has become much stronger and unlike Oden, Momo never learnt anything and he has to face a Kaido much stronger than Oden faced.
no haki luffy beat two logia users.
I am talking about Kaido - the top 5 in the verse, not some random logia, who the grand line are full with. Beating Kaido will make Momo top 5 in the verse out of nowhere, stronger than Luffy, Zoro , every supernova and maybe admirals as well.