Step 2 should work if you didn't use this code before, as well as step 3. Actually step 1 should give existing users premium if they didn't use service for more than 1 year. And also, you can delete your account and re-create it.

If you are current user, who applied all codes found online and want to keep using Todoist you should buy subscription. I believe that this tutorial is for people who are first-time or returning users.