Well, yes, but Sigel is in print claiming that throw doesn't exist, so grains of salt are indicated.
However, in this case I think he's right about using outside WP ("whenever possible", as George Fels says). The thing is that skid happens maybe on one shot in 100. For the average player adding 1% misses does not affect their game significantly, but the errors from using side spin on most shots is a big problem. For Sigel, who might only miss 1 in 100 or 1 in 50, adding another 1% of misses due to skid is a huge deal and worth the complication of using side a lot. Or at least that seems like a reasonable argument.
He also told me deflection doesn't exist and to ignore it. When I asked him, "How so" - he explained how he stroked to eliminate the effects of it. Therefore, it's EASY to take something that Sigel said at face value and laugh at it (implying he's an idiot). However, one might find that's it's possible he's evolved beyond it and developed techniques to negate it in his calculations. If Sigel said throw doesn't exist--- that's his way of saying, "It doesn't concern him the slightest bit because he overcomes it with technique (aiming/english adjustments)."