I have a couple of thoughts on this, based on relevant experience with a large commercial theme marketplace.

First, I think the Theme Team has a very good point that if an exception is made for one theme, they are put into a very difficult position when they try to enforce the rule with other authors. I’ve been there and it sucks. So, in general, giving exceptions is something they should fight against.

Second however, if there is a valid reason behind the exception being asked for, then it’s time to consider whether it’s worth changing the rule for everyone. That looks to be the case here, with the valid question being “should we change the rules to allow for a better onboarding experience?”. To me, the answer would be yes, but I do acknowledge that the Theme Team would have other things to consider, such as the practicality of being to review onboarding tools, etc (if they were heavily based on a JS framework current reviewers don’t have much experience with for eg) – that might be something they’d have to address before they could change the rules, but would be something that could be worked towards.

Third, sometimes what’s considered ‘best practice’ isn’t actually the best thing for the user. That seems to be the case here. If we agree that giving the user a nice onboarding experience is a good thing, then which experience is better – telling them to install a plugin, activate it, and then get the good onboarding experience OR just getting the good onboarding experience (because it’s built into the theme). This seems to be a clear case of needing to allow ‘plugin territory’ functionality in themes because it is actually better for the user. It’s definitely worth re-evaluating ‘best practice’ and rules when it’s not inline with user experience. Sometimes in WordPress land we lean a little too much towards being idealistic over being user-centric.

Forth, this is definitely one of those things that would be good to be in core, so you don’t end up with many different theme authors inventing their own onboarding system (which has already happened to an extent in commercial themes). If core included a robust solution that dealt with plugin dependencies, setting up options, installing demo content / creating pages, etc that theme authors could just plug into, that would would make for a much better experience for end users. Of course that’s something that would take time, so perhaps it’s worth changing the rules in the mean time.