As we all know, following the events of the Whole Cake Island Arc, Luffy’s achievements were compiled, inflating his reputation greatly.
Luffy went to Big Mom’s territory, defeated 2 of Big Mom’s Sweet Commanders Charlotte Katakuri and Charlotte Cracker, completed his objective and returned back safely after turning her kingdom upside down.
Though his capabilities were slightly exaggerated by the press, his achievements were great enough for him to be unofficially titled the “Fifth Emperor of the Sea”.
I found Blackbeard’s reaction to this news interesting and funny at the same time. I’m not sure Oda was thinking about this when he was writing his line, it is probably just a coincidence, but… think about it.
Blackbeard spent 20 years working for Whitebeard until he murdered Thatch, the 4th division commander, for the Yami Yami no Mi, one of the most powerful Devil Fruits in the world that allows the user to create and control darkness at will, making the user a Darkness Human. Teach demonstrates that the “darkness” is a void which devours anything, able to pull in (much in the manner of an actual black hole), and crush his surrounding environment into a pile of rubble.
Then he recruited notorious prisoners for his crew from Impel Down Level 6.
After that he obtained the Gura Gura no Mi Devil Fruit, which allows the user to create vibrations, or “quakes”, making the user a Tremor Human. This fruit is fearsomely reputed to be able to destroy the world, and is considered to be the strongest Devil Fruit within the Paramecia class, having powers no weaker than those of a Logia.
Using his Gura Gura no Mi he acquired Whitebeard’s former territories.
Then in Payback War he was confronted by Marco and only after defeating him Blackbeard was acknowledged as Yonko.
After the time-skip, he and his crew fought against the Revolutionary Army and destroyed their base. It is also stated that they briefly fought against CP-0.
And now he reads a news about Luffy’s new bounty. A 19 year old teenager whose Gomu Gomu Devil Fruit allows him to stretch his arms and legs. Who was destroyed both physically and mentally in Marineford. Who was absent the last two years… and returned and suddenly became “Fifth Emperor of the Sea”.
“Zehahahahaha!! Emperor? It’s still too early for you, Straw Hat…”
Right now people are very mad at how big the gap is between Luffy and Kaido.
But it is important to note that none of the Emperors have successfully defeated another Emperor in a fight. Remember, Luffy has been hyped as the Fifth Emperor based on twisted words in the media. Him not dying in Big Mom’s territory is being seen as a victory. So the gap was always there. We are just getting smacked in the face with it now.
*Theory by fandrfa