And yep, I do instinctively open nearly everything in a new tab

Then what do you care what the default target is? In a reasonable browser, requesting that a link be opened into a tab should open it in a tab even if target="_new" is the suggested default behavior in the HTML of the link. This is how my copy of Mozilla behaves.

I hate it when a link is javascript or something and it doesn't want to act like normal HTML

I was not considering javascript nor indeed anything other than HTML.

So the suggested feature would only affect you when you forget to open the link in a new tab. In that case, would you rather have a new window pop up which you can just close if it bothers you, or have the in-process preview be replaced by a new page with the risk that going "Back" doesn't restore your text from the browser cache and the node contents that you were composing are simply lost?

I don't find that design decision at all difficult (and I particularly dislike target="_new" in most cases).

Further, for this particular use, I'd find jumping between two tabs (only one of which can be visible at once) to be the wrong way to consult a thread while I was composing a reply to it. So, even if I were in the habit of opening links into tabs, in this particular case I'd make an exception and open a new window so I could view the original node while I'm typing the reply.