You’re welcome! Basically, the Marketplace Vendor support portal is the go-to place for requests to the Atlassian Marketplace team that are specific to your account, add-ons or in this case, participation of the Atlassian Marketplace Vendors benefits program.
It’s only for Atlassian folks, right above it - it says:
If you are an Atlassian developer using a test instance of the Marketplace server rather than the production server, set the environment variable MARKETPLACE_URL to the base URL of the test server, for instance:
Daniel is correct - the Marketplace staging instance is only accessible for Atlassian employees.
I updated the tutorial to use the term “Atlassian employee” instead of “Atlassian developer”, as there’s understandably some confusion on whether third-party vendor developers qualify as “Atlassian developers” in this context.