The wording[1] also suggests they request the images even if you haven't opened the email, which obfuscates whether you've opened the email or not. With other services like gmail the images are only requested when you open the email, so it's possible to infer whether you opened the email or not based on whether the image was loaded.

[1] https://twitter.com/rjonesy/status/1401993816001978375/photo...