Greencard holders like US citizens get taxed on their worldwide income. Depending where they are from there might be a tax treaty negating the double taxation. However the country of residence is the one that taxes. If your parents are from the UK they should of gotten a Nil - Tax code to get tax free pension income in the UK, to then declare and pay here in the US.
That income then can be used for the ACA. Your parents shouldn’t be filing $0 tax returns in the US unless they truly do have no income for anywhere in the world.