often times lots of folks make this mistake of over lump summing bitcoin at the top and when there is a sudden bearish market which last longer than expected, they tends to be scared to follow up with investing further to take advantage of the dip.
There is a different between Lump sum, buying at dip and also DCA method, and in lump sum it doesn't targets the dip to invest all at ones on the contrary it can be use at any given price of the Market so if it happens that your plan of lump sum falls during when the Bitcoin price is on dip that will be an advantage for you, in other words any price is good so long as you are lump suming, meanwhile I see lump sum to be somehow related to the DCA method in the aspect of buying at any price but the difference is the nature of there investment, however your understanding about lump sum seem to be channeling on the buying at dip since you are pointing out that they should reserve some of the funds from the lump sum to be use during the dip.