https://www.volkskrant.nl/nieuws-achtergrond/nepnieuws-kost-artsen-onnodig-veel-tijd-sommigen-zeggen-dat-wij-aan-censuur-doen~b1187468/

According to this article, an intensive care doctor at Amphia sent out a warning message to their social network when she first noticed patients getting really sick from covid-19, because at that time people were still brushing it off as "just another flu".

Then someone ran with it and edited the message to make it seem like the hospital was seeing more casualties than reported and losing control of the situation. That seems to be the fake news part they're referring to on their Twitter channel.

As far as I know, the part about the 16 year old guy on life support and a significant number of the treated being under 50 is still true. If you have information that contradicts this, please share!