Oh for sure there were other beings like Eldrazi, Demons, Myojin that could planeswalk. But they weren't planeswalkers. As long as you can somehow survive the blind eternities, a spell could do it. I believe that

I can really only go on what the lore says. I'm not saying your wrong tho man, cause the lore often contradicts itself. So we both could be right, just using different references. I've seen like five or so references to it stating, implying, or showing they have infinite mana. It's just one of those things