Mixbus could be considered to be a commercial version of Ardour with a bunch of extras. That is owned by Harrison Consoles through a licensing deal with Ardour.
That includes (from a licence perspective) a bunch of Ardour code which is also available under an Open Source Licence.
But, although they come from a similar code base and have many similar features and capabilities, they are also different in many ways.
The primary difference between Ardour and Mixbus could be considered to be the workflow: Ardour is more like a modern DAW whilst Mixbus is more like a traditional analogue console complete with a channel strip which is modelled after a classic analogue console.
But the basic code-base of Mixbus is Ardour, so investment in developing Mixbus results in improvements to Ardour.
By the way, I believe that (at one time) SSL were talking to the Ardour team about creating an SSL version of Ardour which would have been branded under the SSL marque, but the project never got off the ground.