Hi Flavio, that's indeed how it works and worked for me. Please note carefully one of the last sentences in that description:
Once you integrate this with your Squarespace site, you will need to remove the Google Analytics (GA) code you have setup currently. Then add the GA code as a tag inside Google Tag Manager.
If you don't do that then your side will not be tracked by Google Analytics. Also make sure you don't have your Google Analytics account number filled in at Squarespace 'external services' because then it will have a duplicate tracking id.
So in the end you only have the tag manager snippet added to your website as explained in the article. Google Analytics will be tracked through the tag in tag manager.
Enjoy tagging! ;) Let me know if I can help along the way.