I'm very well aware that it is open source, but open source does not preclude the possibility of ownership. As you're probably hoping to point out though, it does bear thinking about as it's not entirely clear.

This post musing on the subject is a pretty good consideration of the subject, here are a few of the most relevant points:

According to the standard open-source licenses, all parties are equals in the evolutionary game. But in practice there is a very well-recognized distinction between 'official' patches, approved and integrated into the evolving software by the publicly recognized maintainers, and 'rogue' patches by third parties. Rogue patches are unusual, and generally not trusted

[...]

There are, in general, three ways to acquire ownership of an open-source project. One, the most obvious, is to found the project. When a project has had only one maintainer since its inception and the maintainer is still active, custom does not even permit a question as to who owns the project.

If you read this, and especially the entire post, perhaps you can see where I am coming from with my claim that Steemit owns the blockchain (the steemd project) code.

Here's another interesting announcement from the Steemit blog which could be considered the next one from the one you posted: Steemd is now completely free to use with an MIT License. The license before explicitly forbade anyone to create a new genesis block, that is, to start a different blockchain based on any part of the Steem blockchain code. It's only since this change that other blockchains like Smoke.io are legally able to use this code in their own projects.

The take home points are that open source software does not mean a fully permissive license (although Steemd now has one) and that customs and practice matter when considering ownership.

But just to be clear, yes, anyone can use it, which is probably what you're meaning to say I guess.