Captchas are web page or app based. I am a code geek.

If it hits a web page that is still slaved to the http / https protocol which is centralized.

Examples: What is done on steemit technically has no impact on what is done on busy and vice versa. They both tap into the decentralized blockchain but neither website is the gate keeper of the other.

Once you add a gate keeper you add a level of centralization.

Now whether or not there are already centralized aspects of the steem blockchain or not I cannot really tell you.

I do know the tech is all open source, so if they did centralize it someone could fork it and make a new non-centralized chain. Though yes that would be a large undertaking.