the missile would fall out of the sky in the ramjet stage if the gas chamber is a large carburetor. can't believe we are still arguing about something so basic.Nope since if you inspect the diagram closely you'll find that there is only one fuel injector at the ram intake side, basically the gas chamber is a large carburetor.
The main challenge of fuel combustion is optimizing air/fuel mixture. Creating that air/fuel mixture within the supersonic airflow of a SCRAM jet from the start is a very high technical hurdle so this system creates a fuel rich mixture in the gas chamber and then release it in the supersonic air flow for combustion. This system also solves a big problem with the afterburner model since afterburners requires a large amount of fuel as mentioned by various sources. Probably the main reason why there are no long range missiles that have an afterburner feature.