To buy the dip you need liquid cash sitting around earning very little, hoping the market goes down.

Reallocate portfolios if that makes sense, but much of the “buy the dip” stuff implies good market timing: To do it in meaningful amounts you’d have to pick the right times to not invest, accumulating cash at a lower rate, then you’d have to know precisely when to invest it again, switching it back into the stock market.

Yes. This is one of the reasons 80 stocks / 20 bonds is a common strategy. The rebalancing "forces" you to buy low and sell high.

When stocks dip, your portfolio might become unbalanced at 70/30, so you reallocate funds to buy stock bringing you back to 80/20. Conversely, if stock market soars, you might get to 90/10, and selling stocks would bring you back to 80/20. Performing this balancing is, in effect, "buying the dip"