BB…Companies like DustPhotonics, TowerSemi/Juniper/openlight, Skorpios technologies and colorchip seem to have no issues with their 400g product.

I don't really want to spend any time on this as I am sure there are many here that could enlighten you and may take up the cause but not me. But I will provide this low effort tid bit for you.

Skorpios Technology has been working with ADVA since 2021 so why does ADVA need POET? Read the BLOG on POET’s website. 

"Poet's unique design of its optical engines, with hybrid integration of optical chips and monolithically integrated MUX [multiplexer] and DMUX [demultiplexer], enables us to deliver industry-leading products in a small form factor that is scalable to high-volume production as well as to higher data rates, such as 1.6 Tbit/s [terrabits per second] and 3.2 Tbit/s, thereby enabling much higher bandwidth in a pluggable form factor."