Because the TTC's own trainers need to have way more knowledge as they will be expected to be able to train more than just the operators. Their training is more intense, and some of it may not be applicable to what the operators need. They will be, quite literally, writing the book.
Let's put it this way - one of my friends is high enough on the seniority table that he will be in the first or second class of operators to be trained when the time comes. And he has not been given a date that he can expect to be moved over to Subway Ops in order to begin his training, although he had been given two earlier dates that were later cancelled.
So yeah, I'll stand by my original conclusion about your source.
Dan