Reason why heavy aircraft are allowed to land but not to start
What could it be that allows the aircraft to land safely but not to start?

In the USA at least, to allow takeoff, a runway must be long enough to let an airplane reach takeoff speed, decide to abort, and reach a full stop.  That's longer than just landing.
The more ancient answer is: takeoffs are optional, landings are mandatory.  Refusing permission to land would damage Twente's reputation.
Even more ancient, before aviation, a ship in distress would not be forbidden to dock and ordered to head back into the storm.  (Only because COVID is mentioned, ships suspected of carrying plague are a different matter.  See the etymology of &quot;quarantine.&quot;)

Greg's answer uncovers the Dutch report, which quotes Meiltje de Groot hoping that the 747's could be scrapped (&quot;ontmanteld&quot;) in situ because of lack of demand, because of COVID's reduction of passengers, at Twente because they could land there with no intention of departing.