Here’s how I imagine the setup:

Kylo orders the attack force to approach the castle from a different direction, low over the trees to keep the element of surprise. The Finalizer is waiting above the clouds to make sure no ships escape. When the transport lifts off, Hux or Kylo decides to forego subtlety and make sure nothing escapes, and they blow it up. This involves emerging from the clouds and revealing themselves, but the assault force is already committed to their attack vector. The only thing that changes is that the castle patrons now know they can’t escape, but they still don’t know the attack is imminent, which is why they remain somewhat calm.