So what exactly was confirmed there bud?
The Holy Knights are the clean up squad going against any type of rebellious behavior, with Sabo being
their no. 1 target. Doesn't mean one of the Knights is Sabo's final opponent. Given the narrative, Sabo vs. Akainu is strongly emphasized.
Whether you like it or not, Akainu as the Fleet Admiral is a disposable guy, the outer face of the WG which is worth "slightly less than zero". The Gorosei tell Akainu to shut up while bowing down to Imu, who is the Antithesis to Joyboy, and us also having Blackbeard as a third party, who represents chaos and also being a D, probably wanting to become king of the world like Xebec and Imu being visibly butthurt about both Luffy and Blackbeard. That is a much bigger narrative than being a mindless laptop of the WG. And while Sabo's and Dragon's main target is the WG itself, they will understand that Imu is reserved to Joyboy, while they will fight the WG's top dogs to clear the path, with Sabo vs. Akainu being a great opportunity to make it even. It's like Jozu smacking Aokiji and telling Whitebeard to "go on and bother with more important stuff".