That's now what he's saying and not what the suggestion implies.
He is specifically talking about relays that are both big and public (which I assume he means free of charge to write).
The eventual centralization of the network (as in: most people all use a few big relays) is a phaenomenon that can happen regardless.
It also make sense in order to spread nostr that new users are not hit with a ton of bad content the day they start, or they simply leave immediately.
It is just a marketing idea.
nostr:note1weyc6ds8sajeq39ychr6mk6d7tp65z6tc403ay7h0j36r8lewjjs33l7dy