I referred to 6 October, Felix, because if anyone returns to the UK before then and stays for the remainder of the tax year (183 days), they would automatically be UK resident for the entire tax year 2020-21 under the first residency test. But if you are opting for a split-year assessment, that wouldn't be relevant. I assume Mondial's advice is on the assumption you will be taking the split-year route.
Sorry, I should have said you have until Jan 2022 to file a self-assessment return for the current tax year. It's the 2019-20 return which is due in Jan 2021.