Yeah, but you can still drink the alcohol, just not suffer the consequences... A creature immune to fire isn't prevented from walking into fire. The description says, "When you take a critical hit, you can use your reaction to block it, turning the critical hit into a regular hit and suffering a level of fatigue", which makes it sound like blocking is the action and fatigue is the consequence and not that suffering fatigue is what causes the critical hit to become normal. But I do understand it wouldn't make sense to make that is immune to fatigue automatically immune to crits. How about a human with marathon runner? Would they not be able to prevent a critical hit until they've suffered a level of fatigue in some other way?