A quick google of “uk healthcare s1 abroad rules” immediately brought up NHS healthcare abroad site confirming your position and my being completely wrong.
“If you receive both a pension from the country you now live in and your UK State Pension, you cannot get an S1 form. This is because the country you live in will be responsible for your healthcare.”
The next paragraph likely relates to what I had read re. IIRC the belgian mention, it was probably a poster with a UK pension and small belgian pension living in France. Apologies.
“If you receive your UK State Pension as well as a pension from an EU member state, but are now living in a different EU state, the country to which you paid contributions toward your pension for the longest period becomes responsible for your healthcare.”