True, I should say the author "may" have Synaesthesia. But the spacial appearance of a number line is an absolutely textbook example of it. The interesting thing about his choice of example is that's what he perceives and he assumes everyone else also has a spacial representation of numbers.

Most Synaesthestes don't realise there's anything different about their thinking until they hear about the condition. So, I just wanted to make the author (and others) aware of it for their interest.