Floyd Mayweather wasn't an undisputed champion in the welterweight division, he had all the belts except the IBF belt at that time because Kell Brook was the one holding it until he was defeated against Spence Jr. in 2016. But yes, Mayweather has many reasons at that time to make sure Manny Pacquiao won't have his chance for a rematch because Floyd knows that he won't be lucky anymore.
That's why having a rematch clause is really important especially for a big fight just like this one, an undisputed title is on the line together with the 4 belts. It will just end up like Mayweather-Pacquiao back in 2015.
Well, I don't know how many Mayweather fans are, but we all know what he did with Pacquiao, in boxing there should always be rematches, no matter how strong they are, they should exist, and I didn't like that time there was that fight where Mayweather seemed to be in A speed race in the ring, where you could only see a boxer and a rabbit running, where everything focused on the technical, where many were not left with a taste of the sport and above all very badly the judges, this is something that I did not like and I hope to never see a fight like this again, or by Pacquiao but by a boxer like Mayweather, and Mayweather fans excuse me, but it had to be said.
They already know about that fact and they cannot deny that their idol was once a marathon runner inside the boxing ring against a much older Manny Pacquiao and their fight was already called years earlier when Pacquiao was a bit younger but Mayweather didn't go along that discussion as he knew that a prime Pacquiao will definitely stain his record.