You're reading the percentages wrong.
It's 94% of all the buyers are millennials under the age of 40, there could be two or 1 million of them!
That doesn't mean that 94% of the population is buying, but that doesn't mean the majority of millennials are buying or using crypto, it doesn't mean anything from a global perspective. For example, if 9 people under 40 buy bitcoin and one over 1 does the same you have 90% of the buyers being millennials, but there are still only ten buyers.
"420 million crypto users" is approximately 5.3% of the total population of the world, that's not even close to 80%.
With the mempool clog happening with just 400k transactions, I really believe the 400 million! "users" number is pure bull!