Balloons are open. Most typically do not expand but the excess air just escapes out the bottom. Basically they will rise till the overall weight matches that if what they displace.
There are more efficient balloons that do expand and can attain same great heights. Far more than conventional aircraft even. But that expansion is mostly due to excess material in the construction and little from stretching. Thus the pressure difference is minimal while the volume increase significantly with altitude.
To put it in context, Artemis did many fully destructive tests but typically on the ground. Artemis had and spent an overhaul budget that was likely close to a hundred times that of what SpaceX is spending in today's dollars.
And even better representation, all the fully destructive tests of SpaceX have carried out have costs less than a single successful shuttle launch. And it has a much larger payload.
Even with the destructive tests, of which are planned this way, not only is SpaceX is far cheaper than any past space program, they are advancing fairly rapid.