I have performed many experiments to prove this very point. Tape your thumb and index finger together, and your middle and ring fingers together, and see what wonders you yourself can accomplish without opposable thumbs. At a lecture I delivered in 2006, I demonstrated this concept vividly. With my hands securely in "Dino-Mode", I proceeded to write on a blackboard in a fluid, clear hand; I also used a number of DIY utensils including a hammer.
Opposable thumbs have been proven by the most up-to-date science as non-mandatory components of advanced motor function.
This is cherry picking of good results taken to its dumbest. You choose among the hundreds of jobs the two or three that can be done in "dino-mode" and declare victory. Have you ever asked yourself whether someone could have made the blackboard, chalk or hammer without opposable thumbs to begin with?
In fact, I also wrote on a blackboard with a cast on my hand after dislocating some fingers. And I wrote on the blackboard with my other hand also. Writing on a blackboard is among the easiest tasks you can do. You could even glue a chalk to your wrist and write on a blackboard quite legibly with it. And a hammer is not much more difficult, if it is not a normal 3 kilogram hammer with a 20 cm handle. But if it is, even lifting it is a challenge.
There is nothing particular about using a stone as a hammer. Some species other than humans do it. But from there to making ocean crossing boats there is an evolutionary process of tens of millions of years or more, if ever.
Now, ask the people who have lost their thumbs in accidents whether it is such a small deal. I know one dentist who almost lost his career when his thumb was severed, and could go back to work only when the thumb was re-attached. People who loose a thumb prefer to have their toes removed and attached as thumbs, so they can do manual work again.