I worked one place that had IRIX, AIX, Solaris, and Tru64 on one site, for a company of only 600 people. Each with different service management, shells, low-level (hardware/firmware), backup, etc all to learn. Old UNIX was a fucking shambles, with companies desperate to break things in small and large ways to try and create lock-in to their flavour.
Then the network cards had DIP switches on them and things like thin-net or AUX big network connectors.
This made things we now take for granted a highly skilled domain. Getting your IBM box to talk to the Sun workstation on the other side of the room was rocket science.
The different dialects of UNIX meant that there were niches for those that knew the difference between how to run a 'ps' command on a Sun box to (say) an IRIX box.
Incidentally I am not sure it was about lock in for all companies. IRIX was about doing things in 3D and enabling those applications to work. They had no interest in tinkering with UNIX to make it different for the sake of it. It would be different because they had different hardware and 3D to be standardised.
I felt IBM was different just for the sake of it with AIX. The same with Sun except they dominated in some universities so were the de-facto standard for many people coming through education at that time.
Oh, and compiling open source programs across several of those. Real fun.
Looking at modern distributions I hardly see a difference given distribution specific directories, packages or tooling.