I think we should actually just reopen the question.

Should we close questions where someone is using Linux Mint and not Ubuntu? Yes. But we don't have to be so rigid about it as to close questions that already have clearly useful, upvoted answers that are fully applicable to Ubuntu and are already helping Ubuntu users.

Now, it's understandable why it was closed, since to a extent it is about Cinnamon, and Cinnamon is part of Mint while not ordinarily being part of Ubuntu. Cinnamon is also available for Ubuntu but it's used by a much smaller fraction of Ubuntu users. But it is available for Ubuntu. We have many other Cinnamon questions open on the site. Assuming the question and answer happen to apply equally well to Cinnamon on Ubuntu, I think we should reopen this.

Another benefit of reopening the question is that it would allow more answers to be posted, so if there are any issues specific to this problem for people who use Cinnamon on Ubuntu, new answers could address them.

So, in summary: If we're prepared to keep this on the site because it's helpful to Ubuntu users, then it doesn't need to remain closed. If not, or if there is no consensus about what the question's status should be, we're not under any special obligation to do anything to it. But to an extend I agree with Braiam: While we shouldn't delete this, we should strongly consider migrating it to Unix.SE if we really consider it inappropriate for Ask Ubuntu. Although I totally disagree with the idea that we have to delete useful content to achieve perfect policy compliance, I think it was reasonable and valuable for Braiam to post about this issue on meta.

It's tempting to think of every question as an opportunity to set precedent for the way future question will be handled. There is some truth to this mindset; sometimes inconsistencies can be confusing, and we should consider what will happen later as a result of how we handle questions now. But foolish or pointless consistency is no better than inconsistency. A question is first an opportunity to help all Ubuntu users (and other site visitors), a second an opportunity to help the OP (a close second), and third an opportunity to shape site policy (a distant third).

I think we should actually just reopen the question.

Should we close questions where someone is using Linux Mint and not Ubuntu? Yes. But we don't have to be so rigid about it as to close questions that already have clearly useful, upvoted answers that are fully applicable to Ubuntu and are already helping Ubuntu users.

Now, it's understandable why it was closed, since to a extent it is about Cinnamon, and Cinnamon is part of Mint while not ordinarily being part of Ubuntu. Cinnamon is also available for Ubuntu but it's used by a much smaller fraction of Ubuntu users. But it is available for Ubuntu. We have many other Cinnamon questions open on the site. Assuming the question and answer happen to apply equally well to Cinnamon on Ubuntu, I think we should reopen this.

Another benefit of reopening the question is that it would allow more answers to be posted, so if there are any issues specific to this problem for people who use Cinnamon on Ubuntu, new answers could address them. There seems to be a strong sense in the community that there's something of value to us in this question; reopening it would let people contribute answers that fill any gaps that may currently exist in its applicability to Ubuntu users.

So, in summary: If we're prepared to keep this on the site because it's helpful to Ubuntu users, then it doesn't need to remain closed. If not, or if there is no consensus about what the question's status should be, we're not under any special obligation to do anything to it. But to an extent I agree with Braiam: While we shouldn't delete this, we should strongly consider migrating it to Unix.SE if we really consider it inappropriate for Ask Ubuntu. Although I totally disagree with the idea that we have to delete useful content to achieve perfect policy compliance, I think it was reasonable and valuable for Braiam to post about this issue on meta.

It's tempting to think of every question as an opportunity to set precedent for the way future questions will be handled. There is some truth to this mindset; sometimes inconsistencies can be confusing, and we should consider what will happen later as a result of how we handle questions now. But foolish or pointless consistency is no better than inconsistency. A question is first an opportunity to help all Ubuntu users (and other site visitors), a second an opportunity to help the OP (a close second), and third an opportunity to shape site policy (a distant third).