It is, roughly speaking, a legitimate warning though. If you don't have the legacy add-on, it won't convert any of the existing data over to be associated with your new add-on (options, permissions, etc), which will likely make development of the new add-on a fair bit more difficult (especially considering elements that should be maintained).
Given that upgrades are an option now, it's reasonable to assume that if someone is converting a legacy add-on that they have the old data available for it.