I'm the mod that deleted that answer. I've never noticed this spam text behaviour before, because as Jefromi's test shows, mods don't see the replacement text.
I completely agree that this answer is not spam. In fact, I didn't act on the spam flag. The post triggered a deluge of flags - 4x 'not an answer', 2x 'very low quality', 1x 'spam', and 1x 'invalid flags'. From a moderator point of view, all of these are bundled together, and appear as a single "flagged post" that needs attention.
When a post has multiple flags, I go and look at the full context of the page to decide what to do. Although not spam, I agreed that this was so useless that it could be deleted. A side-effect of taking action by hitting 'delete' is that all the flags are automatically marked as 'helpful', and, presumably because of the presence of the spam flag, the spam-hiding text was introduced.
In other words:
- This was not deleted because it was spam.
- I wasn't aware that spam flags changed the view of deleted answers for high-rep users.
- The auto-marking of flags, while normally a very handy automatic step, in this case did more than was intended.