Every satoshi is not equal. Each one has a different history. At the very least, KYC bitcoin is not equal to non-kyc bitcoin. You can tell because people will pay a premium for non-kyc bitcoin. Therefore, each unit is not a 1:1 exchange with any other unit. Is that not what fungibility is? If you disagree, than I will trade you all of my kyc bitcoin for all of your non-kyc bitcoin 😁