When I say they're gamestack, I'm talking about their client and their backend services and their associated middleware.
Moving a game that is mostly client authoritative to server authoritative is a hell of a lot of work and requires serious rewrites to both the client and the server.
It also requires a lot more compute to handle the back end.
When you go from calculating everything on the front end and just sending the data back to the back end to sending actual controls to the back end and doing simulations, you need to rewrite a significant portion of everything.
It's way cheaper and way faster just to write it in the client, and require the kernel/secured OS to police risky actions to the application.
The last couple of projects I looked at were probably 50% more man hours to make it server authoritative out of the box. Trying to come back and do it after the fact, It's much, much higher.