No need to get a new host or domain - it's just trying to be "helpful" for those who are starting from scratch.
I had hosting before starting symbiostock and had installed another copy of WordPress (I had one already in a subdomain for my husband's blog) into a subdomain. Because I wanted my stock site's url to be top level - not stock.mydomain.com in other words - I bought another domain name and mapped it.
If that makes any sense

For Bluehost, you can use their tools to make a sub domain and install wordpress which makes it very easy. One thing not to do is use the WordPress feature to specify a subdirectory for the WordPress install - it messes up all sorts of Symbiostock URLs and doesn't work.