[johannes1234321]

I think you use the term "plain text" differently from the author of the post. I think they refer to the fact that there is no end to end encryption. Google has access to the clear text of all messages and can index/analyze them.

[MobiusHorizons]

The article does call out plain text email without formatting or attachments. Plain text typically refers to visual formatting, while clear text refers to lack of encryption.