I don't know if that's how dropbox does it, but I could imagine that they have a master key to which normal employees don't have access, you need the founder and a trusted second person to retrieve it.

Thus their statement "Dropbox employees aren't able to access user files, and when troubleshooting an account" wouldn't be too far off the mark, and they can still make the data available to the government, on request and with higher effort.