That is correct. The issue is that there are so many different types of car seats that there is no way to make a system where the car seat locks to the adapter # TH20110713. The bar offers support for the back of the car seat and the strap is used to secure it and the precious cargo to the stroller.
Please note that the following car seats are compatible with the adapter:
Britax B-Safe and Chaperone seats
Britax-Romer Baby-Safe and Baby-Safe Plus II
Chico Keyfit and Keyfit 30
Combi Shuttle... view full answer...