I see that it got your attention since the total supply of bitcoins is only 10,000,000 so that wallet holds the 10% of the total supply.
The total supply is (just short of) 21 million. The current circulating supply is 18.66 million.
There's no source to this assumption and might this theory is true or just a rumor. It might be Satoshi holds a Million of Bitcoins but not on a single wallet.
I wouldn't say there is "no source". There have been a number of analyses of early mining activity, which is where the figures of Satoshi owning 500,000 - 1 million BTC come from:
https://bitslog.com/2013/04/17/the-well-deserved-fortune-of-satoshi-nakamoto/https://cryptowords.github.io/the-return-of-the-deniers-and-the-revenge-of-patoshihttps://blog.bitmex.com/satoshis-1-million-bitcoin/Whether or not these are accurate is another discussion.
OP and some others replying are confusing these two concepts.
Not only that, but people also do not understand that "wallets" are completely fluid, and there is no requirement for a wallet to only include addresses generated from a single entropy source or seed phrase. We could simply import a bunch of private keys (if we knew them) in to the same wallet and create a wallet with over 1 million bitcoin without making a single transaction.