Just to clarify: This letter was about GrayJay, a 3rd party YouTube front-end that -as I understand it- gets its YT content by parsing the YouTube website. This way they can access YouTube content without using their official (or any, really) API. Parsing the site is more complicated and a lot of work to maintain but good for privacy and no restrictions from API terms of use.
Google pretends not to know or understand this by asking the GrayJay devs to honor the terms of an API they actively avoid using.
GrayJay is being developed by a team at FUTO (dot org) that Louis Rossmann is affiliated with. His personal 'Louis Rossmann' YouTube channel is seperate from FUTO and GrayJay, and isn't part of this issue, nor is his personal channel mentioned in the letter.
I hope I remember correctly, I listened to his video while falling asleep ^^