The "Arc approximation" value from the Polygon properties [ 1 ] is what you're looking for. Reducing this value will smooth your polygon arcs - don't forget to repour. Check the documentation for more information.
Side note: It's worth understanding that, based on your target PCB fabrication class and your manufacturer's process, it may not be relevant to have "absolutely accurate arcs": the etching process is somewhat rough. Finally it's a balance to find between file size and (needed) precision.
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