Yes and no, it was created and maintained by one of the developers of steemit but it is just like any of the services that help you sign transactions on the blockchain.
You are not giving out your key you are using it, it never leaves your browser.
This is true for steemit.com as well as the many apps that are being released on the steem blockchain.
When you create an account on one of these services it should be a different password than any of your keys... It's just a tool that allows you to follow the votes of another account.
Thanks so much. That helps a lot