That wasn't that far ahead, especially given where the 'no natural death' cause came in and when the Android Saga started.
Piccolo being an alien was something Toriyama said he thought up when Kami first appeared.
(From Daizenshuu #4)
Q: Speaking of Piccolo, did you think from the beginning of his background as a Namekian alien?
A: Of course, I didn't think that at all (laughs). The Saiyans were like that as well. When I thought up Goku's tail and the Ōzaru, I didn't think Goku was an alien or anything. Piccolo either. Because I thought that up when God came out.
Mostly afterwards, I thought it through so that it would be consistent. For example, there's the chair that Planet Namek's Eldest sat in. That was mostly the same as the chair that Piccolo Daimaō had sat in when he first appeared. It was just missing the skulls.
I honestly forgot when they first said they couldn't revive someone if they died of natural causes. But if it was during Namek, it isn't impossible Toriyama was laying some foundation for what came next (even if he didn't exactly have the whole thing plotted out.)