I Think You Nailed it FJ!!

Mitsubishi Electric will contribute its highly differentiated 400G Electro-absorption Modulator integrated Lasers (EMLs) to the project.

I never knew these lasers were even available yet , all the time I've heard about them they were dots on a future road map. Now they are a reality. Everyone knows the best lasers come from Japan and it looks like Lumentum and Mitsubishi must also agree that Poet's interposer is the best solution/carrier for their lasers from a cost/performance spectrum.

How can a massive operation like Mitsubishi picked a nobody like Poet to partner up with? Because the Interposer Solves their Problems Amazingly Well!
Let's not underestimate Suresh's point about power consumption , all I've been hearing lately is that the AI boom will stall because of the limitations to power capacity and generation moving forward. Any solution that helps save power consumption will be moved to the front of the line.

It's all coming together, Poet is the real deal now, just way too many big players putting us in their ecosystem and not small companies either, but the suppliers to the hyperscalers.