There are a few conceptual things to remember here:
The full quote from that page is
In very broad terms, Ob 1 is easy, Ob 2 is kinda hard, Ob 3 is hard to most and risky for everyone, Ob 4 is really hard and Ob 5 is a fat bastard!
These are very loose guidelines, not hard rules for all situations. Further, they're guidelines for setting Obstacles when nothing else gives you the number. When you know that something "should" be "kinda hard" or "very hard", this part of the book gives you an Ob that goes with that gut feeling. This is not trying to tell you what the odds for actual rolls will be — that will depend on how many dice the player is actually putting into the Test and is fundamentally not the GM's business to worry about, but the players'.
Nature is not the Obstacle, it's a rating that gives you dice to roll. The snake's Nature 7 means 7D, not Ob 7, and 7D in an opposed roll will average 3.5 successes. That makes fighting the snake hard, but not impossible.
Guard mice work as a team, helping each other. They also have gear that give them bonus dice, wises that make them wise and give them bonus dice, and circumstantial bonus dice. They can also tap Nature to get even more bonus dice. The Obstacles are high, but a mouse has a lot of options if they need to succeed.
They just can't use every advantage on every roll, so they have to think hard about when to ensure success and when to endure a setback. Which brings me to…
Even guardmice fail. In fact, in Mouse Guard they must fail in order to advance. They can't ever fail if the Obstacles aren't high to begin with. Choosing your failures is a key part of how strategy works in Mouse Guard.
The example of Liem fighting the snake on pages 109–10 (original printing) illustrates many of these. Teamwork matters, your gear matters, and choosing when to use your scarce resources can turn a very hard Obstacle into an easy success, if you choose well.
A key conceit of the dice system in the Burning Wheel family of games is that you can't win interesting challenges reliably, if you don't care enough to fight for it. And there's no point in rolling if the outcome isn't uncertain and failure a real possibility. Trying is modelled by using up scarce resources when you want to "push" for success; and a real possibility of failure is ensured by never giving weaker Obstacles than the situation realistically would call for.
There's no point in striving and fighting for what you believe in if it's easy, so Mouse Guard doesn't make it easy. It makes you choose. As GM it's your job to set up the Obstacles so that they're real challenges, not guaranteed wins. It's the players' jobs to step up and either rise to the challenge, or fall short and suffer a setback.