Apparently, you can't. You can't even manually mark a single message as read without actually reading it.
I expect that this is by design. If you're done with the message, then you should mark it "Done". If you don't need it right now but what to handle it later, you should "Snooze" it. If you don't ever need the message again, you can always "Trash" it. (All three actions are available when you have multiple messages selected.)
(If you think this is an oversight, you should use the "Send Feedback" feature to let them know. The app is in preview testing, after all.)