First off, the rat’s statement wasn’t a lie, only a falsehood. For a lie the rat would have to intentionally attempted deception rather than just being wrong.

Second, the snake’s statement about not being poisonous was directly connected to “don’t be afraid” with an implied “because”. While technically true, he was definitely attempting a deception. Maybe not by means of a lie, but not in any way less harmful. That would normally be grounds for chopping his hand off, so he’s also abusing the fact that he doesn’t have arms.

That said, the rat was fucking stupid because I the scenario would probably end the same way whether the snake was venomous or not.