A huge leap in millionaire BTC-addresses

The explanation is very simple. As you can see from the data of Glassnode there are plenty of bitcoin-addresses from the early years with the original 50 BTC block-reward with unspent, untouched balances. When the price surged above 20k USD these addresses became millionaire addresses because of the amount of bitcoin in them worth more than 1 million USD.

The early miners and community members were not so aware of the wallets and backups because the BTC was worth literally nothing at the time. A fresh mined address with 50 bitcoin was not worth even a cent and without attention keys, wallets, even devices were lost and for good. This is suggested by the unmoved amount of bitcoin too. Nearly 10% of the circulating supply, 1.78 million coins did not move for a decade. Very likely, the keys are lost, and the coins are unavailable for the owners.