The issue of ads are a red herring, if you don't control the server, and aren't using end to end mail encryption, your emails aren't private in the sense the author intends: someone else's server is parsing your emails and has accesss to the plaintext. If it's not parsing them for ad keywords it's parsing them for spam filtering or for IMAP search indexing or it's a service that supports web mail and so it has to sanitize the messages to prevent XSS.

Honestly this blog post reads like an ad for SlideMail disguised as community PSA. People interested in mail security should choose more secure options than just avoiding server side processing.

End to end is the only real way to go for personal 1:1 messages you need kept private.