You are assuming you can divorce your authoritative server logic from Unity.
You can I believe - your server will end up being a standalone binary with all the necessary Unity things bundled in.
I believe the parent means writing your game logic in pure C# and avoiding the need for Unity entirely server side. For some types of games that’s doable, but when you start needing physics or nav agents or even animation driven logic a headless Unity binary is the only real option.
Or you could use nodejs & socket.io for the MP server which is even cheaper
Not sure how it's cheaper cost-wise, since the .NET runtime is free, but his solution avoids a re-write of game code in JS.