Can Pressflow 7 be a drop-in replacement for Drupal 7?
It's not that there are problems with Pressflow 7, but that there are no significant benefits to it at this time. This is stated in the lines above your quote:


  If your project is built on Drupal 7 you might not need to use
  Pressflow at this time because the key enhancements from Pressflow 6
  were added to Drupal 7.


You could use Pressflow 7 as a replacement, but why bother if there are no advantages? (I haven't actually used Pressflow 7 because of this).