Ironically, plain text emails actually do better for us - the problem is that plaintext emails are often more likely to be caught up by spam filters.
Meaning the 'plain' text is Html-formatted to look plain and there are tracker links and probably tracking pixels too.
Real plain text should be just like something a human being typed.
Also, ignoring the general scamminess of hubspot, the linked article says html mail is MORE likely to be flagged as commercial. The article, in fact, is entirely pro plain text.