Atlassian seems to know nothing about BitBucket account

I have an existing BitBucket account that I use to check on one client's project from time to time. I am invited by the web page to use SourceTree, which I would like to do.. My SourceTree app says I need to "register" and I click on the link to do so..

The Atlassian page I am asked to log-in to seems to know nothing about this BitBucket account. I do not want to lose the privelages associated with BitBucket. I dont actually care about Atlassian at all.. I just want to check the repo I have privs on, on BitBucket. All else is frustration..