I really like the way they done it, simple and minimalist. You don't want to crowd the page with lots of repeated text that will be present on every single post.
It should be easy to change the exact text if it's phrased and easy to style via css if you want it bigger. Remember they're also designing for tablets and small smartphone screens too.
If you have a whole word or even two words it can become cramped quickly, see the following image as an example, which doesn't even have the moderator controls visible. Space is a premium on smaller screens which more people are using.
As for people understanding what it means, I'd rather have it a minimalist + and perhaps an explanation in the forum help to show users how to use it. They'll get the idea quick enough.
Although it could possibly be nice to have some kind of feedback as to how many quotes you have selected after each click. Perhaps a moving bubble with a counter similar to the one that appears when a moderator uses the checkbox to select a post, this could be clickable to bring up the quote review box.
Or maybe that's not really necessary, just thinking out loud.
Adding a a remove all or clear button to empty your quote bank could be nice though. Is the insert into message the only way to clear it?