It seems like the unspoken assumption there is that total enjoyment increases monotonically with hours
No, not really.
In a game that you're supposed to finish in 50h you cant really expect a 40h grind.
But in an open world, sandbox, no ending career mode game, some activities might and will stretch over longer periods of times.
For example if you want to unlock several Guardian Modules - what do you do?
Either relog 2 days at a guardian site (to get all the blueprints and materials needed) or set yourself goal to explore all types of guardian sites doing each site once.
First option is grinding, while the second is gameplay that might keep you busy for quite some time - at least that's how i see it.
Same for engineering. Play the game (missions, combat, trading, mining, planetary exploration), visit some landmarks once each (davs, jamesons, bug killer) get whatever materials you can find, eventually visit a trader or two and engineer your ship as needed (at least G3, maybe more).
Or focus on grinding/hunting HGE, grinding Jamesons 20 minutes, relogging at davs 1h, spending another 1-2 in a Famine center - etc....
The point is, everything can be turned into a grind by players, especially in a game like ED where one is supposed to take it slow instead of fastforwarding everything turning into a grind and getting burned out (or not, but still complaining Oh! The Grind!).