This topic quickly gets murky, both legally and in terms of common sense, but the SD Residency Affidavit is not at all the same as Florida's Declaration of Domicile. The SD Affidavit is a means of compliance with the federal Real ID Act requirements for issuing driver licenses or other official state Id. However, once you get SD driver license and title your vehicles there, you have taken a major step toward establishing SD as your legal domicile as well. Unless you have substantial legal relationships elsewhere, having driver license, vehicle title and mail address in a given state is enough to qualify it as your "domicile" as well.
Conversely, Florida's Declaration form does NOT, in itself, qualify you as a Florida state resident for the purposes of issuing licenses in compliance with the Read ID Act. I told you it is murky, right? ???