This is just an instance shutting down, not all of the Nitter instances. Sort of a deceptive title.
Guest accounts have been removed, they weren't just led to believe that. With real accounts getting rate limited immediately and likely banned, I don't see any path forward for Nitter.
The way I interpret this comment is that all public Nitter instances are at risk of becoming inoperable (at their current scale).
I would say that Nitter as a whole is not owned by anyone, that person is the owner of one of the Nitter instances, in addition to being the primary maintainer of the project as you correctly stated.
I agree with the second part of your comment about all instances being in danger. It is unfortunately a common problem faced by many privacy-respecting frontends (Nitter, Libreddit, Proxigram, Piped etc)
And then there's 'owner' in the software dev sense, i.e. the one that ultimately decides which direction the project takes, what patches to merge etc. is sometimes referred to as "product owner".
The copyright owner of a code is the one listed as such. In cases of multiple contributors, each owns the copyright to their individual contributions. For projects with shared contributions, it's more accurate to say each necessary part has its own copyright owner, rather than a common ownership.
Free software projects do have copyright owners, which can be entities like non-profits or companies. This ownership is crucial for legal standing in copyright infringement cases, except for public domain code, which isn't copyrighted.
It's wise to have a Contributing.md or similar, stating that contributors affirm their creation is original, free from patent issues, and they agree to license their contribution under the project's terms.
Without such agreements, there's a risk contributors might withdraw their code, claiming it wasn't licensed under the project's terms, potentially leaving gaps in the project.
Simply hosting code on a site with a common license doesn't automatically apply that license to your code unless you explicitly agree to it.
Property law itself is a social construction of exactly the same kind.
A single person or group owning a FLOSS project is also a typical, almost universal case. There's always the person or team, and the canonical repository, that represents what "Emacs" is, or what "sr.ht" is, etc. In practice, for everything except legal issues, this is the same as legal ownership.
If they don't get new guest accounts, they're all going to shut down withing 30 days. It's just left over accounts and caches giving you the impression everything is fine.
Don't forget the rolling DoS herds of scrapers having no more ways to do stuff on their own, which are now trying to use the remaining instances - killing even more.
"If nothing changes, all remaining instances will go down eventually: Instances rely on guest accounts, which are valid for a certain time and of which you need a ton to run a public instance. The API for this got taken down and it doesn't look like a fluke this time."
99% of the time I touch twitter, it's to go to a specific tweet or timeline.
Edit: I just checked an instance and it seems to have tweets from the past minute or two, but if the backend accounts that drive them are getting banned, it's probably gonna tank in the upcoming weeks.