The earth equatorial radius is ~6378km. The ISS is just 400km above that, or less than 6%.

99% of the energy is expended just to get into a suborbital trajectory. Its only the last couple of seconds in any spaceflight that raise the perigee from being below ground to being above the atmosphere.

In order to deorbit the station, they just need to lower the perigee enough so that drag does the rest. And a huge space station with huge solar panels has much more drag then a small, cylinder shaped capsule.