>everyone has access to all the cards they want

Top tier commander decks can easily cost over $10,000 I suspect the vast majority of players do not have access to them.

That’s a fair point for Commander. But the GP seemed to be making a universal claim that ELO wouldn’t make sense in any case, even say Standard where the decks are <$500 (or Arena where you can get a meta deck for <$100, free if you have time to grind wildcards).

I think perhaps we could agree on an intermediate claim that Elo could work amongst the pool of players that can build any deck they want, which is a big pool for MTGA, and definitely a smaller pool for physical Standard, and probably very small for Commander.

I wonder if you could decompose the score by playing some games with a fixed deck too? Eg Arena challenges. How much of your overall Elo is just picking the right deck for the local meta, vs raw quality of plays?

That depends fully on the format. Original article doesn't specify afaik, but the only sanctioned commander games I've ever played in a tournament format were limited events

MtG is like car racing. Sure, most people own cars (hi from California) and can theoretically race them. But anyone any good at racing has a lot of money.