'Instant IMC' can occur (GBZ I know what you mean, although you state VMC).
Years ago I had been on a training exercise at relatively high level and then descended the cold soaked aircraft into more humid conditions. The canopy immediately froze over and visibility was zero. The instructor explained the situation and flew on instruments until the frost melted in warmer air lower down.
So this could be a possibility.
After all cold soaked jet aircraft can have ice on the wings in sunny Spain during turnaround if the conditions are conducive to icing, so it's not a wild assumption. Just depends on the conditions - which at the time of this accident I'm not aware?
There can be no proof, as other people have stated, just a possibility.
I can't help but think that the outcome of the enquiry will be just that - a list of possibilities, maybe exacerbated by the state of the aircraft at the time.
KR
FOK