What I see here is two heroku employees (Jon McCartie, wuputah) asking for this community's help to not arbitrarily stamp out questions that are heroku-service related, just cuz they say "heroku". (You know...heroku...in other words, Salesforce.)
Which to me says my heroku colleagues are trying to engage in a meaningful way with questions here on SFSE.
If that produces good useable help documentation here in SFSE that is Salesforce-related, is that not furthering the underlying goal of SFSE?
Being in the "rest of Salesforce not heroku" side of Salesforce, this reflects an internal move that is seeing all of us who's job is developers and platform work harder to be one Ohana internally, and thus reflect that out to our community.
As has been already said, there is a big surface area of configuring the heroku service to work in a project that is will use the service, and not specific to the language it is written in. Why can't those questions be asked here?
The heroku service is part of the footprint of Salesforce platform offerings regardless of whether heroku is connected to an org. Things like private spaces, heroku pipelines, add-on compatibility can operate independently of an org connection. Of course there are things like heroku connect where there will be an org, too. In all the above we have existing Salesforce customers who are beginning to use these. It will be weird for them to be told, "those Salesforce questions have to be asked on StackOverflow."
(begin edit 12 Nov, 2018)
But there is already a heroku community on StackOverflow
The trouble is that StackOverflow has a clearly stated focus on programming. From SO's help: "...working together to build a library of detailed answers to every question about programming." Typically, non-coding heroku-specific configuration questions are closed on SO because they are not about code. In other words, the heroku community there is limited only to programming questions by design. Heroku service and config questions are not welcome.
Alternately, our stated purpose is, "With your help, we're working together to build a library of detailed answers to every question about anything Salesforce related."
The Salesforce technology ethos has always welcomed a mix of code and configuration. Since we already allow config-based questions on this Stack Exchange, it would follow to allow configuration of the heroku service here in the same manner we do with Lightning Platform ("core salesforce"), marketing cloud, commerce cloud, pardot, etc.).
(end edit 12 Nov, 2018)
SFSE is a pleasant community
Let me also advocate for this move on the basis that SO can be intimidating. I would much rather ask a question in this community who I know, and who know me than the wide open often unkind world that is SO. We've tried hard to foster supportive constructive feedback and a sense of respect for all comers. I'm not saying we let every question pass for that reason. Language questions that can be answered independently from the heroku service don't belong.
So yeah...if a question is asked here about Salesforce's Heroku PaaS, why can't they be asked in the Salesforce StackEchange?
In pursuit of the above suggestions, I've modified the heroku tag wiki (pending peer review) here in SFSE to say the following:
The heroku tag in Salesforce Stack Exchange should be used for questions pertaining to the use and troubleshooting of the heroku service itself. Questions on heroku-supported languages should be directed to one of the heroku language tags on stackoverflow.com.