While \this\ is understandable, it is a lot less professional than italic text. It's all about presentation, and this is a shortcut to a better presentation with the same amount of effort.
That probably depends on whether you want to convey meaning by formatting of free flowing text. Professional way to do that is to highlight (highlight, not make bold, italics or whatever) the parts of your email that constitute the tl;dr part, even more professional is to simply write your email such that it's main point is repeated in first and/or last paragraph.
AFAIK there's no way to escape asterisks on HN.
Backslashes certainly don't work. :(
Because you can use this plugin for more than just emails. I've used this in a lot of backend systems that support rich-text interfaces but don[t support markdown.