It doesn't.
The suggestion is literally to get rid of kind 20 and add a "k" tag to kind 1, with a "20" value.
39173 variants of kind 1.
Discussion
The suggestion is to add ["l", "picture-first"] tag. That is compatible with existing clients, yet allows image clients to show only posts that have that label. No unnecessary complexity. Kind 20 will remain a solution to those who seek a smaller audience for their posts and compatibility issues like you noticed between nostr.build and lumina.rocks.
Don't let the door hit you on the way out.