iCloud Keychain encryption

Hi. I know that when I enable iCloud Keychain sync on my device Apple encrypts my local keychain and stores it on their servers. They use end-to-end encryption and data is encrypted on the device and can not be read by Apple.

My question is: what key does Apple use to encrypt the Keychain? From official documentation and third-party articles, it seems that they use a key derived only from my device passcode which is a very short and weak 4-digits string. It means that in case someone was able to break into Apple servers and steal the data it can easily brute force it and reveal the content since it's only protected with encryption with 10^4 of entropy.

Please correct me if I am wrong!