So I'll just note a few things:
1) You can clearly see the Great Depression and 2008 in here, so I'm just going to ignore those.
2) The other really nasty period for market returns was during the 70s oil crisis and subsequent high inflation period.
It also notably marks areas "slightly above inflation" as red, which are not periods where loses would occur (though, yes, the gains would be basically flat). This strikes me as an odd choice and a misleading one that makes the historical analysis look worse than it actually is.
Yet despite that, the majority of that chart shows returns moderately above inflation, sitting in at around the 4% safe withdrawal rate.
I personally don't view this as justifying the claim that individual investors need to worry about market timing. If anything it reinforces my view that they shouldn't because no one could apriori predict the kinds of events that led to the red areas of that chart.