Yes, as for now SDA is using only the default membership provider and the only alternative is to use the Web Site login functionality.
By "...then let me tell SDA to use a specific provider in a multi-provider scenario" did you mean to let the SDA user to choose from the available providers or to let the site administrator somehow configure which provider is used for SDA?
The possibility of choosing from all available membership providers in SDA is a feature already in our future road map and is something we are already working on.
Configuring the site to redirect SDA user to different default membership provider is very good suggestion and we are definitely going to consider it in the future but it’s not in our road map for the moment. This is because the SDA module is just a normal Sitefinity module and like every other module it is relaying on Sitefinity for all matters concerning authentication and authorization. There is no real difference if you call the SDA service or for example blog services.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items