Outside of the GUI, Be is a very thin veneer over Unix. There is, no joke, a class named "BStatable" which, if you know some Unix, is just stat, because of course what their OS kernel implements is just stat.

In the places where there's something entirely different going on, such as networking in earlier BeOS, it's trash and you'd obviously want to throw it away and do what Unix did - Be were doing so. So a hypothetical turn of the century fruit-flavour BeOS is probably much closer to "Unix with a different UI" than you're thinking. NeXT changed nothing there.

It certainly wasn't on the BeOS R5 CD that was available back in the day, or the whole Kits available for BeOs programming in C++.

The amount of work done by Haiku to provide a proper POSIX userspace support proves the point.