We're too used to thinking of these sorts of things as D&D physics. We acknowledge that they don't have to fit normal physics, but expect that they have to fit a sort of fantasy physics. One of the designers (I think it was Jeremy Crawford) responded to a tweet about it by, essentially, saying that's not how they are treating lycanthrope damage immunity (that's where I'm referencing the folklorish thing).
Think of it this way. The curse of lycanthropy doesn't make your body impervious to most forms of damage. Instead, it creates a supernatural cosmic rule or magical state that prevents you from being damaged by mortal weapons. Weapons with a specific supernatural significance (in this case magic or silver) are explicitly excepted from this rule and can damage as normal. Suffering damage from other sources is not covered by this rule at all, and therefore they are not immune.
If you can find a way to get around the weapon rule, (such as by using natural phenomena) they will be damaged as normal. If you go too far and turn it into a weapon wielded by mortal hands (which in game rules generally means you are making at attack roll) it falls under the supernatural rule of protection by the lycanthropic curse.
It's a change from how prior editions have treated it, but I kind of like the feel engenders. I do agree that it causes some consistency issues with regards to why lycanthropes have not become Lords of All Monsters.