The box which is displayed is more or less a standard template.
MathJax is not enabled on every site, and not on every of them the mhchem package is enabled by default. There have been various changes to the site, some of them very recently (like Ch-ch-ch-changes: Left nav, responsive design, & themes and others), and we (moderators) have not tracked all these changes. As such, there might be an opportunity to make a (real) feature-request to include such a thing.
I did not find any possibility to edit this, so we need to elevate this. There is certainly some flexibility as there is a link that point to the rather extensive How can I format math/chemistry expressions here?:
For reference, this isn't the first time it comes up: Spreading awareness about using \ce{....}
The help centre did not always work as smoothly as it does now, see Error on Help Page (Boi did it take time finding that again!)
How to continue?
Maybe it is worth taking the time to revisit the whole thing again. I thought I could turn this post into an actual feature request, but then I realised it is already 6 months old, so I guess a new post would stand a better chance. (And I guess it would be a good idea to wait out the coming elections.) In any case, the request should be phrased like one, for example:
Mention the mhchem syntax \ce{...}
in the "How to Format" box displayed when writing a new question.
[... reasons ... reasons ...]
When that gets a good amount of votes, but no reply, we (the moderators) could use different communication ways to get a final verdict and settle this issue.