Toyota Yaris catalytic exhaust error and E10

I have a 2017 Toyota Yaris with only 90K on the clock. The engine light has come on with a catalytic exhaust error code (P0420) just a month after its regular service.

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The mechanic did a cat clean it but the error came back a few days later.

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Now they've told me the problem was I'm using E10 fuel and that damages the car and I need a new cat. I can't understand how a car that is supposed to be designed for E10 could end up with massive issues so soon into its life due to E10. I also did some searching about catalytic errors and apparently they're usually a symptom not a cause of issues, so I need to know the cause otherwise a new cat will break in the same way.

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I don't know heaps about cars, but the E10 explanation doesn't make sense. Am I missing something here?