The point here is that a formation may only summon daemons as (before) it activates. So the key question is whether or not the troops being transported are part of the transport that is activating.
As a consequence, when the formation starts in an air-transport it may not summon before the transport declares the air-assault.
Your point about daemons not being listed among the units that may be transported is also key (and deliberate) - would you want to be that close in a cramped transport
This does cramp their style a little when assaulting, since the daemons are placed before the formation moves.