Not quite. The source package has to be packaged by someone, then dpkg knows what to do with it.

A Buildpack instead takes source as it is. You use Maven? The relevant buildpacks recognise pom.xml and know what to do. Bundler? Ruby buildpack sees the Gemfile and takes responsibility. And so on.

I as a developer can send the same sourcecode I have in my git repo or on my local filesystem, with no additional effort to prep it. The buildpack does all the work.

Putting it another way: it makes sense to create a debian source package buildpack. There are apt buildpacks in the wild, I expect they'll be revisited.