Market timing both works and is bad advice for the average person by definition (it takes being way better than average to be a successful market timer).

Also you're competing with people who have access to insider information (even though they're "not supposed to"), so it's kind of tilted against the average person; though not impossible to still come out ahead.

It's similar to poker. For the average person playing poker is a -EV game because of rake, but if you're skilled, poker can be very much +EV.