You guys were right. I asked DH Support, and I guess the wiki had updated since the last time I read it. Now it says near the top of the wiki:
Key benefits of using a Let’s Encrypt SSL certificate:
It's free – Anyone who owns a domain can obtain a trusted certificate for that domain at zero cost.
It's automatic – The entire enrollment process for certificates occurs painlessly during the server’s native installation or configuration process. [b][i]The renewal occurs automatically in the background.[/b][/i]
I'd put a link to the wiki in, but according to the dreamhost newsletter that came out today, DH is giving up on the wiki and going to a blog...
Thanks all for your help!