They did more than just switch to the history API. During that period, if anything went wrong, you saw a blank page and, of course, robots saw only the generic launcher HTML instead of any content.
Now, here's what a tweet looks like without JavaScript enabled:
(The main difference would be that things like the search & menus are either interactive controls or simple links to basic HTML forms depending whether JavaScript loads)
During the hashbang era you couldn't use a page without a full rendering ending. Now, however, all of the content is available with fairly rich markup:
