Well, it is still decentralized. The author still has a copy of the repo and can always upload to other servers.
Here more about GitHub suffering from state-censorship: https://en.wikipedia.org/wiki/Censorship_of_GitHub
GitHub also censors any content that they deem stolen (such as the NSA Malware / EquationGroup releases) and content that they deem containing PII-data (such as the public LinkedIn crawl of people stating they work for ICE).
GitHub makes judgment calls on what to censor: They deem a project name offensive, they deem some piece of code stolen (unless the NSA admitted to them it was legit, they had no confirmation), they deem publicly available data crawled from the web PII or harassment instigation.
Unless you do not classify these examples as censorship, it is what it is.
The objection stems from the fact that censorship is always a dominant issue raised in discussions about platforms for sharing and, while it's important to keep it in mind if you're the type of person who might share controversial or objectionable content, that is type of sharing is the minority.
If we were in an oppressive regime like China where state mandated censorship is the norm, then it would be a much more pressing concern. But for most of us, we're not under an oppressive regime and decrying the platform for censorship is on par with complaining that reddit won't let you dox people or flicker banned you for posting kiddieporn.