Kimmel does not have inside information into CBS finances. No one here is suggesting he does.
Picture it this way in radio terms: Kimmel is a heritage morning host on an iHeart morning show in a Top 5 market. Colbert is a heritage host on an Audacy station in the same market on a similar format. Both clusters have similar ratings. Both shows have similar budgets.
Kimmel isn't going to know Colbert's finances line by line. But there are going to be some major similarities.
And---again---Kimmel and Colbert share an agent.
For those who don't know, high-powered agents like James Dixon make it their business to know the financials involved so they can perform the best negotiation for their clients. They don't fly blind. And they share that information with their clients so their clients are on the same page.