Buying Bitcoin all at once in large or large amounts is also not a problem for investors or companies who have a lot of money and I have also seen this in several investment companies that are very brave to do this, for example, like Blackrock and Microstrategy today where they very often buy Bitcoin in large quantities and the DCA they do is also in larger amounts rather than in smaller accumulations like investors who only have limited capital. However, what you need to understand here is that a strategy like DCA is not bad at all even though the amount of Bitcoin purchases can still vary greatly for each person because it is also always adjusted to our respective abilities.
These two companies mentioned by you have huge amount of money but they are adopting DCA in investment. Maybe this company has a lot of money with which they buy a lot of bitcoins at once but they are constantly buying Bitcoin. They sometimes buy more bitcoins and sometimes they buy less bitcoins. MicroStrategy recently announced that they will raise another two billion in capital to purchase Bitcoin. They are currently buying small amounts of bitcoins but their goal is to buy bitcoins regularly.
Every investor should buy Bitcoin regularly and hold their investment for a long time, just as these companies regularly buy Bitcoin. If someone has the money to buy more bitcoins then he will buy more bitcoins and if someone has the money to buy less bitcoins then he will buy less bitcoins. Still he should buy bitcoins regularly.
There is no much to puzzle on this, right from time to time the direction to accumulate Bitcoin has always accord on this buying gradually if you can't afford lumps sum that most people felt it's those that have money that invest in Bitcoin, the DCA strategy which has given room to all kind of investor even the so called lumps some required investor to to keep accumulating while holding for long time in order to get a good profit at the appreciate time because this regular buying or accumulating for me can covers some lapse most the one investor buys when there is ATH because the investor keeps buying regularly he can also buy at a period of ATL when the dip is experience and both situation can be merge to attain the required profit when the actual holding period is put in place.