Indeed, I doubt that Newton would have pronounced Principia with a hard 'c'; Allen observes that in England, schoolmasters only started to observe meter and the hard 'c' and 'g' starting from the mid-nineteenth, and only in parts.

I wanted the commenter to elaborate, since it seems to me that the most likely reason they remarked on its pronunciation would be that they discovered the reconstructed pronunciation of it and thought that it was correct to the point of being knowledge of how it ought to be pronounced, rather than as one of several ways of pronouncing it to communicate it.

Ranieri contends that many casual speakers end up observing only partly observing the hard 'c' and 'g' and little of the other differences and end up with a hybrid "not even worthy of importing". https://www.youtube.com/watch?v=IjcX3MVSdyA