It shipped with Microsoft apps, so quite possibly them?
(The article doesn't mention this later became "Multifinder" and included with System 6 iirc.)
Multifinder could be thought of as a spiritual successor to Switcher but it shared no code with Switcher and took a different approach to solving the problem of allowing a user to interact with multiple running applications.
Erich Ringewald, the author of Multifinder, and Andy weren’t done with this problem at, or with, Apple. I got to work both of them, Erich at Be and Andy at General Magic/Eazel.