I don't really like to play into arguments that rich people are more privledged, since anyone could come into a situation in which they have lump sums available and they are willing to dedicate some or all of that lump sum to bitcoin investing. For example they could 1) win the lottery, 2) inherit money, 3) have had been investing for 10 years and building their investment portfolio little by little (so they are not exactly rich, but they have been engaging in good savings/investing practices), 4) received a bonus at work for some kind of job they carried out (expected or not expected these kinds of things sometimes can happen to even less wealthy people), 5) they recalculated their various funds (emergency, reserves and float) and unexpectedly, they come to the conclusion that they have an extra $2k that they are able to invest 6) they received a gift from friend/relative/acquaintance and/or 7) a variety of other possible reason they could come accross extra money, whether that is some extra from their DCA amount or maybe it is a more modest amount
Exactly anyone can actually use lump-sum strategy depending on the individual condition though , and sir JJG have already given some wonderful example already , like winning a lottery, recieving a bonus at work for some kind of job they carried out etc. You can only say that the rich has more advantages when it comes to lump-sum purchasing, like most time due to having alot of cashflow, the money most rich are using may be higher than an average Man.
Example let's say I want to start investing right now in bitcoin and I have a monthly income of 1000$(it's assumption figures) and I decide that I want to invest about 300$ from that amount into a weekly DCA investment which should be about 75$ weekly invested in bitcoin and then I also had some cash from my savings that I also wanted to use to invest in bitcoin maybe to give myself some kind of head start and the money was about 3000$ and I decide to use 1500$ to invest right away, that is what a lump sum buying would mean.
Lump-sum payment is complete opposite of DCA, you aware that DCA is the purchasing set amounts of Bitcoin at regular intervals, whether the price is high or low. While lump-summing is the act of going all in at once, just like the example I gave about the $3000 and all that. Most people that are financially stable may use lump-summing to start their investment to have some good head start in their accumulation without it affecting their regular life style or tampering with their investment. But if an individual who's not financially stable start his accumulation using lump-summing without any proper planning he or she may endup using the funds that are being meant for their emergency (which known emergency funds) in order to execute a nice lump-sum purchase. So such individual won't be able to sustain his self without any emergency funds which may lead to he or she tampering with their investment , and won't be able to execute his long-term plans, selling their investment in an premature state. It is always advisable to invest according to your financial capability, by starting investment with some nice DCA strategies, so that you can set a proper plan for their investment.