When you check out a document for editing, the last modified field is also updated. So logically the rule should work. Are you saying that the notification is sent to the wrong person in your tests?

Yes indeed , our customer tested it, but the notification is send to the previous editor instead of the "checked out to" user.

The property "Checked out to" is hidden in the config.

Okay, then I suppose that support article should be reviewed and tested to make sure it works. It does mention that "Checked out to" has limitations but not sure what was the idea behind this if the suggested solution doesn't work either.

FYI, the article has now been removed from the Support Portal. The support team tested it again and found out that you are indeed correct about the proposed solution not working.

FYI, the article has now been removed from the Support Portal. The support team tested it again and found out that you are indeed correct about the proposed solution not working.