The point of the article is not that defederation should be used as a management tool, but that it can be very effective in protecting the fediverse from becoming a monopoly play among big corps only.

See, you said it yourself, "don’t see tier 1 ISPs de-peering each other", "don’t see major providers blackholing major providers", "major players don’t de-peer other major players", you talk about big players only not blocking each other, but that misses the point of small players being blocked out by the big ones.

quote from the article:

Blocklists for email exist and are shared across services – and blocking is often pre-emptive, not based on suspicious behaviour of that server. Sure, email is an open set of protocols, but it’s also highly restricted by large companies and not at all open to either smaller providers or individuals.

This isn’t just an abstract issue: I know of friends who have had to abandon email servers they ran themselves, sometime literally on a box in the corner of a home office, because the big corporations that dominate email simply wouldn’t deliver anything they send.

That's the risk, corporations are not stupid, they see the potential in the fediverse right now but they have the nasty habit of "embracing, extending - not always extinguishing but making it almost impossible for new competitors to enter the market".

They could totally ruin the fediverse if left unchecked.