If you are sending out an email, you can use whatever url form you like?

This is talking about links to third party sites, not your own.

use a unique url for each email

As your reader, I might not actually want you to know.

If I release a video and send an email newsletter at the same time, which one caused the traffic increase? Should I invest in making more videos of sending more emails?

If you insist on knowing, include a different url in both that goes to the same place and use your damn server logs. You don’t need google analytics and whatever.

Isn't putting in a different query string "including a different url that goes to the same place"?

Isn't this functionally the exact same?

presumably you control the urls you are sending in the email. As a result if you want to use query strings that's fine. The issue only arises when you use query strings to implement tracking on someone else's site instead.