Yikes! But could a balloon carry an EMP that big? I'm not very familiar with the tech, but surely it would have to have quite a bit of size to it in order to influence a range that large.
Don't know, but they can carry pretty heavy payloads if they're big enough.
I'm pretty sure the altitude required to have straight line of site to the lower 48 is several hundred km, I've no idea how high these balloons could go.
But they could gauge how the US reacts and make notes - a series of smaller EMPs strategically placed (and from lower altitudes than discussed above) would cripple the US power infrastructure for a long, long time. I've read that the large transformers used in distribution substations generally have lead times measured in months or even years - now imagine having to replace thousands of those, along with hundreds of thousands of local transformers...
Anyway, this is 100% conjecture. I've no idea what they're using that thing for.