Questions and answers about natural solutions are on-topic if there is a link to sustainability, e.g. if the natural solution has less GHG emissions, less packaging, less pollution, etc. I agree that a natural solution in itself isn't necessarily more sustainable than any other solution. However, proving the reduced environmental impact of any solution, natural or otherwise, is far from trivial and can be very complicated.
I think it would be going too far to require any question about a desired natural solution to provide proof beforehand that the proposed solution is really more sustainable. OPs ask questions because they don't know enough and are seeking expert advice. It's up to those experts to refute any false premises in the question and indicate whether a natural solution is indeed more sustainable or not.
In my opinion questions about natural products should only be put on hold or closed if it is very obvious that the proposed solution doesn't improve sustainability.
The main criteria for any answer is that it should provide at least a partial answer to the question or alternatively show why premises in the question are incorrect. Answers that do not do this and/or provide no explanation whatsoever can be flagged and removed. Good answers explain things clearly and back up statements with references.
The Q&A at hand:
I agree that the link with sustainability in the toothpaste question is not immediately clear. The OP did try to explain it in the comments, but that hardly helped its case. However, if the OP would argue that he wants to reduce plastic packaging (toothpaste tubes) for example then I think it would be on-topic.
So in my opinion the question is a poor one, but not bad enough to close it.
The answer you are referring to does answer the question, so there is no reason to remove it. However, it could be much improved by indicating in what respect home-made toothpaste is better (or not) from a sustainability point of view.