Beanstalk public key is already in use

I talked to Beanstalk team and they said in order to use their deploy feature with a BitBucket private repo as a submodule I can add their public key as a deployment key to my BB repo.

Their public key can be found at http://pubkey.beanstalkapp.com/beanstalk_rsa.pub\\
Too bad I can’t use it as a deployment key in my BB repository because “Someone has already registered this as an account SSH key.”

Well, I’m pretty sure someone really already registered this key - is a public key, after all. This limitation doesn’t make much sense.