Seems like a good partnership to me imho, granted I've never worked with Canonical's support.
I don't know if that's indicative of the typical support contract experience with them, but would happily repeat the experience.
Don't read evil intentions into the word "fork", it's really the only way Canonical can promise any sort of support. They won't be developing the fork to compete with Blender, they'll just be supporting it and maintaining it. Trying to do that in a place where you have no commit authority would be impossible.
However, sometimes the differences can be sharp, such as when Mozilla disagreed with Debian's patchset[1] (it was around trademarks, UI assets and branding). IIRC Mozilla asked Debian to stop calling its package Firefox, so Debian - perhaps vindictively - chose "IceWeasel" instead. Good times.
1. https://en.wikipedia.org/wiki/Mozilla_software_rebranded_by_...
> The support offering will include Blender LTS releases across Linux distributions, Microsoft Windows, and macOS.
They're not using quilt to do this. They're using git. At least, that's how I did it when I did it for Canonical.