I think you're thinking of a different threat model from these users. If you're concerned about breaches by malicious actors, then yes this defense makes sense.

However, if you're worried about data mined from tracking your personal behavior, which is what the users here are worried about, then it makes sense to spread your data out. Traditional stores are not going to send each other your transaction history to build a profile of interest and personality, and each store won't have a complete enough history or even the expertise to mine it.