An answer from HubSpot Sales Management: "In order to use Custom SSL, you have to purchase the add-on - it isn’t packaged with anything else. If it was communicated that it could be added strictly by purchasing a marketing package (such as Marketing Hub Starter), we apologize for the confusion, but it has to be purchased as an add-on.
“And when I took a look at the community post, it seems to be mentioning Marketing, CMS, and Service because those are all of the hubs that have a content piece that can use a custom SSL (landing pages, website pages, and Knowledge Base). Definitely understand the confusion there though.”
So the answer seems to be that if you own a package that needs SSL (e.g., Marketing, CMS, Service) and you want a custom SSL certificate, you will need the Custom SSL Add-On.