This is incorrect, according to Apple’s developer docs, which states that only the public key is sent to servers. They emphasise this as a benefit: your key can

This is incorrect, according to Apple’s developer docs, which states that only the public key is sent to servers. They emphasise this as a benefit: your key can’t be leaked because servers only ever receive your public key. The private key never leaves your device except to be encrypted and synced to your other devices using iCloud Keychain.