You can dismiss unread posts so that they don’t show up on your unread list and dismiss new posts so they don’t show on your new list (so this is what counts as Inbox:0 on Discourse–I’m inbox:0 here and on my email accounts, where I regularly archive email messages without reading them, removing them from the inbox), but you can’t mark them read without reading them.
Discourse gives users credit for reading and posts credit for being read. If the user didn’t read the post then they don’t get credit for reading it. If the post isn’t interesting enough to read, it doesn’t get credit for being read.