Thermador does not interchange two burners for a grill or griddle.

A two burner grate interchanges with a dedicated grill or griddle pan.

On built-in griddles there is a burner that runs the length of griddle so there is no cold spot in the middle. Here you still have two burners and a cold spot in the middle.

On built-in grills you have an open flame or an infrared burner running the length of the grill. No open flame no outdoor grill type results. Infrared is its own type with a unique type result. Neither is achieved with Thermador. Still a cold spot in the center.

Thermador gives you same result as putting a griddle or grill pan on top of two burners. The benefit is you get a custom grill pan/griddle for maximum useable space.

Your Capital link did not work for me so I don't know what Capital range you are looking at.

The Thermador you are looking at is what most people call a rangetop not a cooktop.

IMO

1a Capital Culinarian rangetop,
1b Bluestar RNB rangetop

way down here somewhere Thermador rangetop.