Thermal layers vary, so their ability to hide a sub should & would realistically vary. If the game models that I do not know.
The speed by which a sub changes depth varies as well, there are a number of variables that need to be understood. There is no one speed for changing depth, so in one instance an hour would be fast in another, an hour would be slow. So I am sure Dick OKane was speaking about a specific situation and not the "rule".
Frank