High secondary skills will, indeed, speed up your training. Or to put it in another way, players with comparatively lower secondary skills will train their primary skills slower.
This is not the case! One fact does not include the other as it is not inverse. Unless you have this as a certain fact by BB. Training speed should not slow down because it is higher, it only states that lower secondary skills will speed up. Stating that primary skills will slow down is an induction you make from the first statement and it is not correct, unless the previously stated is incorrect.
The precise definition, the way I understand it: if you're training skill X, trainining will be somewhat slower if all other skills are comparatively low (as compared to the trained skill), and faster if all other skills are comparatively high (as compared to the main one). What slows down and speeds up depends on what you use as a basis for your comparison.
By extension, this works both ways: when you train a single skill, and everything else is left behind, then you train progressively slower as that skill runs further and further ahead of the other. Likewise, if you train a low secondary skill, you train faster since the primaries are way ahead.
"I don't know half of you half as well as I should like; and I like less than half of you half as well as you deserve."