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In the nonpublic ticket, I learned that Dropbox has separate logins for the app, this Community (which is described as "social media"), and the support ticket system. This makes the process more confusing than necessary.
I sent the log file, but it was of no use; the support ticket agent could not figure out why Dropbox continued to start up and could only suggest that I go back to Microsoft.
This was a new installation of Windows on a brand new machine. Microsoft has already responded by making clear they have no responsibility for how third party apps behave. There is a legitimate use case for apps being able to embed themselves in various places in the startup process, and it is up to each app to provide effective user controls that actually make the desired changes in the app. If the app does not respect the user's choices, that's not Microsoft's fault; it's the app maker's. This really should be an FTC issue for violations of what Dropbox is telling the user!
I launched a boot trace to find the cause of the problem and I noticed that the corresponding option in the preferences of the application does not disable the startup at all. It has the only role of requesting a "quiet close" of the Dropbox processes during the startup phase. It also explain why you still see Dropbox in msconfig even after unchecking the box...
That said, Dropbox create a key for its own startup, hidden in Windows registry, the only option to fully disable Dropbox at startup is to remove it from there.