You have trouble understanding it because it is an absurd claim based on weak guesses at best. Basically many years ago someone hypothesized that certain bitcoin blocks looked to be following a "pattern" and based on that he concluded that they were all mined by Satoshi. Ever since then people have been claiming a lot of early blocks were mined by Satoshi hence the 1 million BTC balance sum.

The funny thing is that from time to time an early adopter comes along and reveals that the early mined blocks were mined by them and moves the coins. Here you can see a lot of early block rewards that were already moved by their owners:
https://blockchair.com/bitcoin/outputs?s=block_id(asc)&q=is_from_coinbase(true),is_spent(true)#f=is_spent,block_id,time,is_from_coinbase