A more significant development, I think, would be if online services let you keep your various accounts permanently in synch. That way you could write a post on one platform and know that your followers on another platform would be able to see it.
Sadly that still wouldn't fix the problem that you have to visit each platform to see responses from users that don't similarly syndicate their own posts. That might lessen those platform's concerns about implementing this automatic synching feature, though, and take them a step closer to being properly federated.
A good point he had is that this kind of thing would, seemingly counter-intuitively (but it makes sense) strengthen the incumbents and stifle both innovation and competition.
Innovation -> Interoperability has a (maintenance) cost and would probably quickly devolve to "lowest common denominator functionality" while raising yet another barrier to entry for new companies
Competition -> Incumbents could pretty much just exploit new companies as "market research" and gobble up all their features and data if they deem the experiment successful, at no cost
I'm not sure how strong those arguments are, actually.
* Interoperability is feature just like any other, and the difficulty of implementing/maintaining it must be a fraction of the difficulty of competing with the network effects of entrenched online services. Indeed, I would hope that interoperability would pay for itself, in terms of effort, because of the number of users that can migrate to the new service, as well as being a selling point in itself (since people would be reluctant to sign up to yet another incompatible silo).
* I think incumbents don't need to rely on interoperability to do market research and copy features of competitors. It's true that Facebook would be able to see private posts on Mastodon instances that it federated with, but I don't know what useful data Facebook could gain from that which it couldn't gain from A/B testing its own huge user base. If anything, I would expect Mastodon instances to gain more from this exchange, because they are gaining access to the bigger pool of data.
Sadly that still wouldn't fix the problem that you have to visit each platform to see responses from users that don't similarly syndicate their own posts. That might lessen those platform's concerns about implementing this automatic synching feature, though, and take them a step closer to being properly federated.