If one is familiar with POSIX and why it is how it is, one knows that the reason that POSIX has both sessions and process groups is that "sessions" pretty much descend from AT&T System 3 process groups and "process groups" descend from BSD process groups; so one would indeed have expected that there were two separate ideas of "process group".
Things that this history does not cover include System 5 Release 2 "shell layers". Shell layers multiplexed virtual terminals onto a single physical terminal, preceding its obvious successors screen by somewhere around three years, and tmux by about quarter of a century. The popularity of tmux and screen nowadays shows that the BSD job control mechanism did not entirely kill off the vision of shell layers, as I recall people thinking it had at the time.
Things that this history does not cover include System 5 Release 2 "shell layers". Shell layers multiplexed virtual terminals onto a single physical terminal, preceding its obvious successors screen by somewhere around three years, and tmux by about quarter of a century. The popularity of tmux and screen nowadays shows that the BSD job control mechanism did not entirely kill off the vision of shell layers, as I recall people thinking it had at the time.