The PIN and key derivation wraps the actual encryption key that's stored locally in the device or secure enclave, not the actual secrets that are stored in the provider's cloud. The actual wrapping keys are random 256 bit AES-GCM keys. This approach works because the secure enclave provides measures against bruteforcing and tampering.
There is some controversy that I can't find an explanation for in any whitepaper, specifically here: https://support.apple.com/en-us/HT202303 where it reads "(...) this data remains secure even in the case of a data breach in the cloud. If you lose access to your account, only you can recover this data, using your device passcode or password, recovery contact, or recovery key." because that implies off-device use of the PIN, so those measures are lost. There's no further explanation that I could find about that. Some previous discussion about that particular point here: https://news.ycombinator.com/item?id=33897793&p=2#33900540