The idea is that a downturn then comes late enough that it doesn't have enough effect on your portfolio. I share your concern. I wonder if this is one of those cases where it fits the historical data available, but maybe there's not really enough data to be certain enough it will work in the future.
It's also based on a 30 year period. Proponents here throw the idea around without mention of age. Some of us potentially have 40 or even 50+ years in retirement, especially if they beat the average in remaining life expectancy. If you do the glide path for 10 years and those 10 years have so-so returns, you still have 30-40 years left and I think you are still subject to sequence of returns just like a new retiree has.
Most people are careful not to apply the "4% rule" to an extended retirement, but I see no such caution when talking about this glide path strategy even though it was specifically targeted for a 30 year retirement, just like the 4% rule. And while the 4% rule can be easily modified for a longer retirement by dropping it down to perhaps 3.5%, I don't see a similar easy adjustment for the glide path. Do you extend the glide for 20 years? Start with even lower equities? Stay low for 10 years and then start increasing? Start high, then drop low, then back to high?