Okay, I actually dug up the book, and I think I'm starting to understand how the authors are using "whence."

When math books use "where," they use it to clarify variables of some given formula. So you have something like:

Let X = A + B and Y = A + C where A = 2k, B = 2k + 1, and C = 2k + 1

The "from where" definition doesn't make much sense when you substitute one for another, but it does make sense when you reverse the order in which the formulas are introduced.

Let A = 2k, B = 2k + 1, and C = 2k + 1 whence X = A + B and Y = A + C

In other words, "where" links a main formula with smaller formulas that clarify on the main formula while "whence" links smaller formulas which are then used to build a main formula. Most math books I've seen would phrase the statement like

Let A = 2k, B = 2k + 1, C = 2k + 1, X = A + B, and Y = A + C

or

Let A = 2k, B = 2k + 1, and C = 2k + 1, and consider X and Y such that X = A + B, and Y = A + C

I guess that's a mystery solved. I really wished I figured this out while I was studying for my finals instead of over a decade later lmao