These are different strategies in accumulating Bitcoin. Some investors use DCA strategy, others prefer buying the DIP strategy, while some DCA and also reserves funds to also buy during the DIP and some accumulate with lump-sum (Rich folks). All these approach are decisions made individually with what suits our bitcoin investment goals.
This assertion has been corrected @jay juanGee of which I completely agreed with him that anyone can actually come in to a situation in which they have lump sum amount available and make such decision of utilizing the lump sum buying strategy in to Bitcoin investment such as one could (1) win a lottery (2) received a bonus at work (3) or an inheritance money, just few to mention which of this doesn't necessarily or exactly make them rich but out of their personal decision to lump sum buy after taken care of their living expenses and provisional emergency funds, what am emphasizing on is that the idea of the lump sum buying is not only subjected to the rich. But however, the logic is that every investor must choice a strategy or strategies that will suit him or her.