I have now looked at my file and paperwork from the time (late 2019). Regulation 6 from the HMRC Tax Credits Manual has been updated (since the Pandemic) and the following paragraph has been added:
"Regulation 6 is subject to regulation 7D (Ceasing to undertake for work or working for less than 16, 24 or 30 hours per week)."
From my records, and a paper copy of Regulation 6 I have (which had last been updated in August 2019), that paragraph didn't exist. It has since been added.
I think that is why I was advised I was only required to notify them if I didn't return to work after 28 weeks. At the time, it wasn't relevant anyway because within 4 weeks of becoming ill and stopping work, I made a claim for ESA and I was required to notify HMRC Tax Credits Office of that, just as the link you've provided regarding changes affecting tax credits makes clear is such a circumstance:
"start or stop claiming benefits for yourself or a family member, or those benefits change"
