Yes, that statement struck me as well. Why would Whitefield wish to restrain the preaching of predestination? Did he think this was not of vital importance in the understanding of God's character?
It may be a harsh truth of scripture but there are many more truths in scripture just as difficult to comprehend. Did he bring this up only because it was a known dispute between the two camps?
I think it's the other way around. Wesley was the Arminian. He was going to preach against the doctrine of Predestination, and Whitefield was imploring him to keep silent.
The Wesley sermon that Whitefield was protesting can be read here.