It’s equally valued, if someone says a snake is not poisonous they are usually meaning that it’s not venomous, as there’s very few contexts where people are worried about the risks of eating a snake, and very few are poisonous.

So yes conversationally the rat is right as he knows the concept is mis implied.

However, it’s also worth pointing out that, the story doesn’t tell us whether the snake is venomous at all. He’s drawn with fangs so, maybe, but it’s entirely a moot point because snakes don’t need venom to kill a mouse.