The MiTM allows the attacker to itself request and receive a certificate. You're right that it doesn't allow the attacker to tamper with (other than to block) t

The MiTM allows the attacker to itself request and receive a certificate. You're right that it doesn't allow the attacker to tamper with (other than to block) the legitimate site's own attempt to receive certificates, but the legitimate site also can't block the attacker from completing the validation process.