This is a popular argument, just one small problem with it: the 4:3 displays of old (640×480 et al) were also "wide" rather than "tall". So by this logic, there would have never been a time where horizontal tabs (or indeed, a horizontal taskbar) would have "made sense".
So I think it's reasonably easy to see that this is not and was never the actual driver behind this decision. It's completely retconned.