Holding 400 bitcoin for 12 years is almost unimaginable for most people because financial discipline on that level is rare the temptation to sell during dips or even after small gains is strong and many investors lack the patience to see through an entire cycle let alone more than a decade that is why stories like this stand out they show what is possible when someone has both vision and iron discipline.
It's number of bitcoins stored in a single Bitcoin address and we never know that the person owns that Bitcoin address only has that one or also has more bitcoins in the other Bitcoin wallets and other Bitcoin addresses. It is a common challenge for all on-chain analytics companies that they can not identify total Bitcoin owners as it is an impossible task.
I agree that it's unbelievable if a person only has 400 bitcoins in one wallet and that address did not move it in 12 years and did not sell any fraction of 400 bitcoins during those years. It's super challenging mentally to go through 4 market cycles and did not take profit either partially or entirely with 400 bitcoins.
https://charts.bitbo.io/yearly-candles/