"Just curious as to where you found out that Poet provides the light for these interconnects for this UEC stack, and important consortium of companies."

By the nature of the hardware. The "stack" is what is referred to as the "OSI Model" of networking.
(OSI Layers Explained). The TCP/IP Layers are a condensed version of the OSI (TCP/IP Layers Explained)

The Physical Layer, is, as implied, the hardware connectivity layer that is used to transfer data between devices.
It's where digital bits (1's and 0's) are converted into electrical or radio signals for transmission.

The interposer, from my understanding, converts the electrical signals into light for transmission over optical cabling (Tx) or the reverse (Rx).