Could it be that they had tickets which restricted them to using those services?
......or perhaps (and quite rationally based on the recent abysmal services) they felt it safest to get on the first available service, albeit crowded, as a later one may well be cancelled/massively delayed?)
I'm not sure there is any (advance) tickets which would restrict to a specific service between Plymouth and Cornwall, or at least not that I've seen.
The 1752 was staffed and boarding long before either of the other services arrived, which to me was a positive sign that It would run. It seemed like it was for the sake of 10 minutes earlier to get home to travel in crowded conditions which I imagine from past experiences to be hot and uncomfortable.