MMM seems to essentially advocate market timing
Rebalancing isn't timing. You can do it at fixed intervals if you want, or you can do it based on relative percentages, but the point is maintaining an ideal AA, not timing. You don't have to know what the market is doing next to rebalance, and you don't have to care.
Cash on hand? MMM has made a point of NOT even having any cash, instead preferring 'springy debt'. He claims to scarcely even own bonds! So taking advantage of low prices sounds great, but after a bit of inspection, it seems that to take advantage of them you have to hedge much harder than MMM lets on. Prices are low for a reason, after all: nobody has any money!
What am I missing?
The fact that you're working, earning more than you spend, and investing more all the time. And if the market takes a shit, you can work even harder at saving so you can invest more, you think about giving up things you normally wouldn't, because the likely returns are extra high. Or you might make slightly different decisions about employment, based on how much each option would enable you to invest. You might dial up a side hustle so you can buy more. And so on.
I have actually considered holding my FT job longer if the market tanks, just so I can invest more. Most of my FT paycheck is already being invested, and I won't need the job for long, but I'm not miserable here, and I'd gladly stay longer to keep investing a few grand a month at discounted share prices. I have always planned to overshoot my minimum FI number just to have more to give away, and that'd be a prime opportunity to make a few months of work count for millions someday.