I can't see how that can possibly be true. NAC has leased the Q400s to Flybe 2 and so it is hardly likely to seek to detain an aircraft which it has just delivered! It is a standard term of a lease that the lessor confirms that the aircraft is free and clear of all liens anyway.
Of the other creditors from Flybe 1, none has the power to detain an aircraft and if it did seek some form of legal process to do so, I'd expect that to be overturned PDQ.