That’s why that’s not the standard advice. It’s index stocks hedged with bonds. If you manage to buy at the “top” of your entire country’s economy, you’ve got bigger problems at that point.
Buying after any previous crash, instead of before, gave you 10-20 years of extra retirement. If you waited 10 years in cash for a crash before going all-in, you came out ahead.
Does that assume that you're able to perfectly time the investment with the market bottom?
Only within a few months. Buying soon after the crash versus buying soon before the crash.