Aren't the diatoms usually present in the final stages of cycling? Seems like waiting for the tank to finish cycling would somewhat eliminate the need for the oto in the first place.
While diatoms commonly appear around the same time in which a tank normally nears the end of the cycling period, the occurrence of the diatoms is merely coincidence and not caused by the tank cycling.
Cycling, as you know, has to do with the tank building up sufficient bacteria to convert ammonia to nitrite and then nitrite to nitrate.
Diatoms appear due to silica in the water. For some reason, new tanks (or older tanks that have been cleaned and restarted) have issues with excess silica in the first month or two after being started. This resolves itself naturally without any intervention.
In some cases, a tank can be jump started to cycle quickly. For example, I moved a large canister over from my 90g cycled tank to my newly restarted 75g tank. That brought over enough bacteria to instantly cycle my 75g tank. However, I still got diatoms in the 75g a few weeks later which eventually went away a month or so after that.
So while the two events tend to occur around the same time, one does not cause the other. They are both caused by the tank being new.