I really like what this theme was trying to accomplish. In essence, it is solving an issue that WP Core has always had, by introducing an onboarding process.
However, I agree 100% with the decision of the themes team.
There are clear guidelines about what themes should do and what they should avoid. There is a clear distinction between plugin and theme territory.
These rules are not arbitrary… They have been painfully curated and created after a lot of thought. All rules and guidelines in place were created after someone abused a “hole” in the system.
The theme guidelines and rules are upheld exactly because we need a healthy ecosystem. If something is allowed for one theme, it must be allowed for all.
As Carolina mentioned in a comment above, approving the theme would open the gate for all kinds of abuse and security issues.

Some people here mention that the theme review process is broken… It’s not. What is really broken is people’s perception of what a theme is.
What needs to change is not the themes review process… If a feature is missing from Core, then we can’t just start adding random features to themes – no matter how valid or useful these features are.
That’s why we have plugins.

Ideally, an onboarding framework would be part of Core, and I wish we could have that – but I don’t know if there are any plans for something like that anytime soon.

Until then, everyone should respect the guidelines & rules; they are there for our protection.