Yeah, I ran into this issue myself a year ago. However it’s really not that complicated to register as self employed and issue one or two recibos verdes before closing the self-employment activity again. With that you can request the NISS online, and with that in hand you can request the EHIC.
But this really only applies if you are living in Portugal, so I don’t see how someone who doesn’t live in Portugal would qualify. After the rules changed last year I’d say an EHIC requires you to live in the country, it doesn’t “come included” with a GV residence card alone.
Just to clarify—“work income from abroad” is a very concept narrow, essentially work you perform while outside Portugal. If you are performing work (earned income) while living in Portugal (even if remotely for a foreign legal entity), you should still be registered either as self-employed or an employee in Portugal, which would make you liable to SS and hence eligible for a NISS.