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If John needs to send 0.1 BTC to you as payment to your product, and he wants to retain his BTC ownership confidential, and the transaction goes something like this...

1fdfsefesf..... ---> send 0.1 BTC to 1hsjhjkwa..... (your account)
---> send 999.9 BTC to 1lkskakkd... (not your account)

... then doesn't that mean John has at least 999.9 BTC left in his savings?

Even though John may not want you or anyone else to know he has that much but the transaction has just given away info of his savings.

So how can anonymity really be sure?

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Bitcoin isn't anonymous in any rigorous sense, and the developers never claimed it was. See for instance https://en.bitcoin.it/wiki/Anonymity. If someone has been telling you that it is, then indeed that is "fakery".

However, the specific issue you describe is pretty easy to avoid, practically speaking. If John doesn't want you to know his total holdings, then he should split them across a large number of addresses. For instance, there is nothing stopping him from spreading his coins across 10,000 addresses, each containing BTC 0.1. (There will be some transaction fees but clearly he can afford them.) Then he can send you coins from just one of them, and you won't really know about the others. If he wants, he can use a mixer or similar service to further obscure the fact that those addresses were funded from a common source.

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