I don't think he's opposed to it, he just recognises that in practice federation and open protocols impose large hidden costs that are typically undiscussed and unrecognised.
Federation between WhatsApp and Allo would face some obvious problems: features that don't work the same way or don't work at all, for instance. Moxie's point is that federation is just less important than it once was because there's no real lockin due to the pervasive use of phone numbers as identities. There's no real reason to use Allo to send message to a WhatsApp user or vice versa given you can just install both apps for free anyway.
Federation between WhatsApp and Allo would face some obvious problems: features that don't work the same way or don't work at all, for instance. Moxie's point is that federation is just less important than it once was because there's no real lockin due to the pervasive use of phone numbers as identities. There's no real reason to use Allo to send message to a WhatsApp user or vice versa given you can just install both apps for free anyway.