This is again in part running in to the "range of player ability" question, though, where there's a really large overlap between the capacities of a single top player and an entire mid-sized mid-effort group that there isn't really a clean distinction. There's plenty of content which is effectively group-only in the game already if you're not a 1-in-10,000 player (which a disproportionate number of forum posters are, sure)
Most Thargoid Interceptors were intended as group content (even the Cyclops, originally). Yes, with a fully engineered ship and a lot of practice at the right technique, some people could take on Hydras solo, but most people couldn't. So is that group content or solo?
(I'm not a top-quality combat player by a long way so even normal High CZs I'd treat as group content - sure, with ammo synthesis I can get through them and my ship's in no actual danger throughout, but they're just a slog, whereas with two people they last a fun amount of time and have more interesting challenges than the Mediums so I wouldn't pick anything else)
The scale of Powerplay effort required to shift a system by one state is multiple weeks for a top 10% player - or several months work for a top 50% player. It's intended to be and usually is done cooperatively but there's nothing strictly stopping one player doing it alone and there are a few players per Power who can do a system a week single-handedly (though literally only a few). Multiply that up to the scale of an entire 750-system Power and even the top individual's effort is a rounding error.
It's certainly technically possible to haul an entire Orbis' of cargo in the 4-week deadline, but most players do not have the spare time or patience to do that and it's clearly intended to be a group thing. The bigger multi-system projects like the bridge to Orion or to Lagoon are very definitely major collaborative activities.
For the sake of defining "group content" I'd say that if something can be done by a lone player (however tenacious or skilled they might need to be) it doesn't really qualify as group content.
The reality, of course, is far more nuanced.
Any activity can be "group content" if there's some benefit/enjoyment to be had from a group of people doing it together.
The point of the post you quoted is that trying to fulfill both objectives (creating group activities and making all activities accessible to lone players) IS going to limit the potential of those activities - especially the ones aimed at groups.
ED is in a fairly good place right now (IMO) so maybe this is a good time to go all-in on creating some activities exclusive to groups rather than just giving players some new toys and saying "Hey, now you can do the same stuff you've been doing on your own in groups".
It's kind of like showing up at a tennis court with a big bag of racquets and balls and saying "Hey guys, now you can play tennis in teams!"
Chances are, the players are just going to ignore the bag and carry on with their game.