unless you load the 3pcc/mpp Multi-Party Platform firmware on it, it will only work with Cisco CallManager systems
No, that is not true.
There are so many users of CP-79xx who have integrated them to Asterisk and 3CX phone system.
I, for instance, have several 7965 & 9951 at home and connected to FreePBX.
Do you mean this: https://www.youtube.com/channel/UCAj4_i8LhB8vMsyRAcPyh6A