Kaidou being a deterrent is a fact, Admirals are not allowed to attack Yonkou territories by design, and other Yonkou and pirates never dare step foot on these territories
Except that Aramaki didn't say that he wouldn't have come because the Marines are not allowed to randomly pick fights with Yonko, but he simply said that Kaido kept "enemies/predators" away, a generalized statement to emphasize the strength Kaido held.
As a matter of fact, Luffy was officially declared one of the four Yonko alongside Shanks, Blackbeard and Buggy and Aramaki was aware of this, holding Luffy's new bounty poster, yet went to Wano to "clear the mess" and even asked the Marines not to tell Akainu so he can make it a surprise. In Egghead, the marines even pointed out that they needed to wait for command to attack Luffy who is now a Yonko, despite Luffy not being in his own territory.
So how exactly did Aramaki's statement about Kaido refer to marines being unable to attack Yonko by default unless they got a command, if Aramaki specifically targeted Yonko Luffy behind Akainu's back?
We all know Aramaki was sent there so Momo and the Scabbards could demonstrate that they are capable of standing their ground against enemies without Luffy helping, plus randomly hyping up Shanks and Film Red by having Shanks making Aramaki retreat.
The fact another admiral, Akainu's equal to be precise, is now a Yonko commander (out of his own conviction), pretty much seals the deal. The hierarchy is set here.