The phrase "not your keys, not your bitcoin" refers to the idea that if you do not hold the private keys to your bitcoin wallet, you do not have full control over your bitcoin.
This is because the private keys are what allow you to sign transactions and transfer ownership of your bitcoin to others.
In the case of an MPC crypto wallet, the private keys are not held by any single party but are instead computed jointly by multiple parties. As a result, it can be argued that no single party has full control over the bitcoin in the wallet and that the phrase "not your keys, not your bitcoin" does not apply in the same way as it does for traditional bitcoin wallets.
However, it is important to note that MPC crypto wallets are still subject to various risks and vulnerabilities, and it is always important to carefully consider the security and reliability of any crypto wallet before storing significant amounts of crypto assets in it.