Obviously carrying a large mountain like this in this danger zone was impossible. But still Rein and his party were able to think about carrying it because of the special properties of the Huguocite rock. It was a fantastical rock which could radiate a special energy frequency which could give monsters and blessed humans a slight power boost.
This special energy frequency was not unlimited. So if a very powerful ability used up a large amount of this energy then the size of the rock would decrease. It then would again need a month or two to get back to its full size again.
So the only way for Rein and his group to carry this rock was if they could consume enough energy from it to make it small enough to be haulable. But with how large this rock actually was, it would be extremely hard to consume most of its energy. At least it was impossible for just the three of them.
