All lighter-than-air vehicles depend on having some atmosphere to displace. To reach 200,000 feet, the vehicle will have to be very large to have enough volume to displace the very weak atmosphere. However, that atmosphere is still strong enough to create drag. At some point almost certainly below orbital velocity, the drag will equal the thrust available from electric propulsion and the vehicle will no longer be able to accelerate.

I didn't see any reference to the electrical source, either. Electric propulsion requires a lot of power to operate, from kilowatts to megawatts depending on the thrust levels required. They say the vehicle would take 5 days to reach orbital velocity, so you're talking about many day-night cycles. Solar power isn't an option. What's left? Nuclear? Fuel cells? Either of those options would probably be pretty heavy, and I can imagine the reaction of the enviros to a flying nuke.

There's another issue that I didn't see addressed: reentry. How are they planning on getting a 6000 foot vehicle back out of orbit without burning up or collapsing due to aerodynamic forces. As we saw so tragically last year, this is not a trival matter. With such a high surface area to mass ratio, such a vehicle would probably slow very rapidly when it encountered aerodynamic drag. That means high g loads, which mean higher structural weight.