I have been down this rabbit hole a few times.
So can we state for certain that there was no turntable before 1889, no, I would bet a reasonable sum there was not.
I also know that logic can sometimes fail but having being in the Como Roundhouse with a live locomotive find it hard to believe that people basically lived, eat, slept in there when it was being used to turn locomotives.
I do not have my sources to hand but my recollection is that there was not much activity in the early 1890's so no great reason to install a turntable,
Often it seems we can find out what happened, why seems harder.
There is certainly a strong suggestion that they cleared out the debris of the fire to allow continuing operation, before putting in the new turntable, which means one was there then.
Everything I have read suggests there was not much done on a whim, costing were done, a lot of correspondence before anything was decided upon. I have seen mention of a fire that destroyed records but forget where and when this was, installing a turntable in a pit does not seem much of an issue back them, so I think there was a reason not to and then when it was decided to do so a reason not to do it then and later the need re arose and they did it.
They had a mile rate for running a locomotive, they would have known how much it was costing them.