Re-read the link, there's an option to completely reset the Important settings and start fresh. Click on the "Don't use past actions to predict which emails are important". That should turn off the feature where Google is attempting to decipher what is important by analyzing your email both incoming and outgoing. Then you get to flag things as Important manually.
When Gmail reads your email it figures out what is important and tags those emails. It's looking at the words you use and the words in email you receive and making word associations. It is constantly learning about you every day. It calculates connections between you and others and topics of interest, etc. It's using artificial intelligence and machine learning to determine what is important to you personally.
Email tagged as important is then filtered into that Important folder. It's not really a folder, it's more like a database query. Gmail doesn't really do IMAP in the traditional sense, it is database backed email. Gmail can talk IMAP (mostly) and Apple Mail reads the IMAP protocol. But there are all sorts of reasons why Gmail will experience bizarre behavior if you treat it like a traditional IMAP email server. If you were to say try to migrate to another email provider and you dragged and dropped stuff in Gmail to get it into a desired folder structure you could end up with everything in one folder. This is because Gmail folders are not actually folders. Literally, it's faking IMAP. You will have a better user experience if you use Gmail in a browser rather than any other IMAP application such as Apple Mail. Or you could choose a different email provider besides Google. It's your choice.
