That's a misrepresentation of the novels, really. These aren't alternate history at all. Nor are they "speculative fiction" as often mislabelled as well. Because Stephenson is very succesful in SF literature, people seem lost when it comes to describe his novels written outside the genre.

The Baroque Cycle is a very researched plain historical trilogy(nothing "alternate" about it - it takes more than fictional characters to fall in the alternate history genre), focussed largely on history of sciences (maths and physics, primarly, and centered a lot on Newton, Leibniz and the London Royal Society) with a dose of economic history, and a prequel to his techno-economic thriller Cryptonomicon, and exploring much the same themes in the Baroque rather than in the contemporary and WWWII eras.