In such situations, I recommend including a more specific explanation in the edit summary when resubmitting the edit. Also, in general, I recommend including specific explanations in edit summaries for code edits or any other edits that must be evaluated primarily on their technical merits. Although you referred to a comment on the post, that comment does not actually establish that the edit it advises is correct; it merely reports that the recommended change worked for someone.
The reason that edit suggestion of yours is correct is that
/dev/mapper/sda5_crypt is the device that contains the root filesystem. Normally it is mounted as
/ but it is being mounted as
/mnt/ubuntu in the live environment, apparently for the purpose of creating
/mnt/ubuntu/etc/crypttab and running commands that depend on it.
/mnt/ubuntu/etc/crypttab will be
/etc/crypttab when booted (or chrooted) into the system. Therefore
proc must also be mounted under
Obviously you cannot include that entire paragraph in your edit summary--or, even if you could, it would perhaps not be the best idea. But you could say something like:
The root filesystem is mounted as /mnt/ubuntu (due to "mount /dev/mapper/sda5_crypt /mnt/ubuntu") so dev, sys, and proc must go directly in /mnt/ubuntu, *not* in /mnt/ubuntu/boot.
(Note that I have omitted code formatting in the above quote to show what an edit summary might look like. Edit summaries don't support code formatting. Normally you would of course use code formatting.)
You've already resubmitted the edit. Although you could re-edit and change something else trivial in the post in order to change your edit summary, I don't recommend doing so, because that would potentially make a situation that is already confusing even more confusing. Instead, we'll see if reviewers are able to convince themselves that your edit is really correct.
So far, as of this posting, one reviewer has rejected the new edit suggestion, and another reviewer (me) has approved it. It needs more reviews to determine its fate.
Update: As Byte Commander says, your resubmitted edit is approved.