That never happened though. He said his blade knows no restraint which literally means even if he doesn't actually try, Luffy can still die which he would have if like 1 million people didn't try to save luffy.
Depends on the translation and how you interpret it. Others say his blade knows no restraint or simply "I won't be merciful". Doesn't mean he would go all out. As it stands, Mihawk has portrayal over Shanks as the WSS. Why would he have a speech about the desire for Zoro to surpass him, even train him, if he could just fight Shanks? Why would he be scared of fighting Shanks while at the same time training someone he believes will beat him too?