It’s something that people with an intermediate-level knowledge of finance say.
The beginner viewpoint is that cash-in-the-vault is an asset because having cash is a positive thing.
The intermediate viewpoint is that cash-in-the-vault is a liability because it implies that the bank owes money and owing money is a liability.
The advanced viewpoint is that cash-in-the-vault is indeed an asset, owing money is a liability, and conflating the two concepts is a bad idea because it prevents you from doing proper accounting.
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