Having skimmed through this article, the article actually sucks. Like, actually.
Why would game demos set unrealistic expectations? That's only the case if they spent 90% of dev time in the first 10% of the game and use that as the demo. This happens, but this is bad design, this is not a case of a demo hurting a game.
I remember playing the Metal Gear Solid Peace Walker demo a long time ago and it did not spoil the story (which is a big aspect for MGS), it did not take away from the experience, and it introduced me to some of the fundamental mechanics of the game.
A more recent example is perhaps the demo of Enotria: Last Song. Do you think I know the lore and story just by playing the demo? No. Did it introduce me to the core mechanics of masks and a few other things? Yes. Was it good to get player feedback and fix bugs? Hell yeah. Sure, you might not like the game, but that doesn't change the fact that the demo had a successful impact i.e. it gave both parties (us and the studio) a significantly better understanding.
Don't put peak content in the demo and it will result in it actually playing like a demo.
Design the game with the demo in mind, don't make it an afterthought. Demos are very valuable and I'm sure lots of gamers pirate games before buying them, instead of actually buying them, because 2 hours for a Steam refund just isn't enough! I spent around 6 hours playing the Enotria demo (just for 1 complete playthrough). Why so many? I had to familiarise myself with the mechanics, switched between keyboard and mouse multiple times, tried to see why I was getting frame rate drops despite my RTX 4070 renderring at 1080p 120 fps.
**Game demos do not hurt a game! ** Abusing game demos as purely a form of marketing by making false promises or setting false expectations hurt the game.
This is not exclusive to a game either. Consider a recent release like Elden Ring, absolutely massive map, they did not try to over-sell it. They said it's around 30 hours of content iirc. You can hurt this game with social media, with interviews, with false trailers, with many things.
TLDR; Angry old man starts raging about "back in my day we had game demoz".