Steam is definitely a walled garden, it's just not the ONLY garden. Epic has a fenced one with mediocre gardeners, the publishers have their own planter boxes, there's the untamed wilds of the pirate scene, etc.
It's (thankfully) not like the Apple/Google Play duopoly. You can still by and large sell PC games however you want, but a lot of devs/publishers choose Steam both because of its marketshare and because of its useful features.
That is true for all consolidated markets. Everyone could in theory create a service to compete with Amazon. There is really nothing preventing people from doing that apart from the fact that it is extremely difficult to penetrate that market.
In this sense those are walled gardens, but because of the nature of those markets. E-commerce, telecommunications, datacenter, banking to some degree are all examples of hard to penetrate markets.
The original article goes on to explain that going head to head against solutions that are consolidated in these market are a waste of time and money. Better to find and exploit a niche that is still untouched.
It's (thankfully) not like the Apple/Google Play duopoly. You can still by and large sell PC games however you want, but a lot of devs/publishers choose Steam both because of its marketshare and because of its useful features.