Did they ignore all other clues like flying below VLS, and "Check gross weight" messages?
As
Shaka Zulu has surmised, they weren't flying an Airbus, so didn't get those messages. The aeroplane *was* providing plenty of other clues but...
When I said "performance", I meant in-flight performance as in the old ZFW/GW mixup. Luckily the thrust reduction was done on gross weight otherwise you'd have read about it in the papers.