I had the same recollection as you, but just looked through a Turbo C book I have here and couldn't find any reference to its big brother. A bit of Googling later, and it turns out Borland C++ was Turbo C/C++'s successor. The releases went like this: Turbo C 1.0, 1.5, 2.0, Turbo C++ 1.0, Borland C++ 2.0, then onwards. So Keen may require features that only appeared in 1991 onward.

Microsoft took more of the approach I think we're remembering. For example, BASIC PDS 7.x being the more professional version of QuickBasic 4.5.