That is the sad part about that , in which we are not sure that if those 1 million wallets are truly owned by each person or there are multiple of those are handled by one person , though the milestone is achieved and we must look in to the good side of this .
It is part of a truth and they did not hide the truth. They even are not able to identify what are addresses belong to one owner. Addresses can be connected that they belong to one wallet if they have transactions in history that help explorers to connect those addresses.
I agree with you that the writer should have a note about it and without note, it is like they intentionally hide the truth but we can not exclude possibility that the writer even does not know about this technical thing.
You can use LookintoBitcoin.com with some charts for addresses with balance > x+ amount
BTChttps://www.lookintobitcoin.com/charts/addresses-greater-than-0-1-btc/