I mean, it is their planes, they can do whatever they want with them. If the UAE wants to get the most out of an exercise where China is likely to provide glimpses of training with better planes than anything in their arsenal, they should send whatever they think are their best planes.
F16 block 60 is not a cutting edge plane. China itself does not even actively procure new planes of that performance level anymore.
If UAE wants to get value out of the exercise, that's what they would go for. If it is just a political statement of alignment, they could do army or navy exercises. Imho UAE might genuinely be looking into improving their air warfare in the medium to long term. Hence, this is a prelude of what will come and also the UAE's test towards what terms and platforms China would be willing to offer.