As I wrote they misunderstood the 'in Unix everything is a file' meaning. For them, a file was a disk file. The fact you could access other mechanisms with the same file API didn't touch their minds. Moreover, there was no need of three different processes, they could have shared data in memory directly. And locking at the whole file level they serialized more than was necessary.
They just added a layer of complexity to follow blindly some Commandments.
Even funnier, they didn't catch the issue because they mostly tested each process separately, so disk access contention didn't happen.
Biting the hand that feeds IT
