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.
It is not only rich folks that buy bitcoin with the DCA strategy; even a newbie can start his bitcoin investment with a lump-sum strategy in some situations. For instance, if a newbie starts accumulating bitcoin late, he or she can decide to do a lump sum buy on bitcoin to accumulate a good quantity of stash before the bitcoin price is too high, and he or she will continue accumulating bitcoin with the DCA strategy.