The comment broke the grace period.
According to ex-CM Shog9's answer,
Edits will be rolled into the previous revision if the previous revision was created by the same author and none of the following conditions are present:
The previous revision was created 5 minutes or more in the past
A comment has been added to the post after the previous revision by ...
I think you got the most important part right about reviewing edits: if you can't tell whether the edit is an improvement or not, skip it and let it be reviewed by someone who can. This applies to any edit on any SE site, not just formulas.
Having said that, formulas are used in a context, at least in good answers. That context often allows you to ...
I don't think formulae are really special in this regard:
If you see an edit but you cannot tell whether it actually improves the post or not, skip it. Whether that's because you're not familiar with the formulae or the topic of the post in general is immaterial.
If you see an edit that corrects a typo, approve it.
If you see an edit that actually alters ...