Where the author talks about imaginary numbers being completely "made up" and suggests you shouldn't bother with trying to understand them, I think that's selling them short.
Imagine, if you will, trying to explain to the ancient Greeks the idea of a number that can't be written as a division of integers (the irrational numbers). That would have seemed completely "made up" to them, but we don't really see them that way, they just "are". That concept is has since become normalized, in terms of everyday concepts (like the area of a unit circle). Similar situations arise with fractions or negative numbers to some indigenous tribes, etc.
I guess what I'm saying is that complex numbers only as fictitious or imaginary as any other set of numbers that we otherwise feel like we have a good handle on.
Imagine, if you will, trying to explain to the ancient Greeks the idea of a number that can't be written as a division of integers (the irrational numbers). That would have seemed completely "made up" to them, but we don't really see them that way, they just "are". That concept is has since become normalized, in terms of everyday concepts (like the area of a unit circle). Similar situations arise with fractions or negative numbers to some indigenous tribes, etc.
I guess what I'm saying is that complex numbers only as fictitious or imaginary as any other set of numbers that we otherwise feel like we have a good handle on.