The reply you received from Anker contains both half-truth and totally inaccurate/misleading information.
Using a USB-C to USB-C cable and a USB-PD (Power Delivery) Power Adapter can achieve the fastest possible charging rate supported by your iPad Pro - this being 18W (i.e, 1.5A @12V). USB-PD compliant devices can charge a higher voltages than the standard USB 5V, achieving faster charging.
When using a USB Type-A to USB-C cable and a standard USB Type-A Power Adapter, the maximum charge rate is 12W (2.4A @5V). The majority (if not all) Aircraft USB charging systems using the standard/universal 5V. It will obviously take linger to charge at 12W than at 18W charge rates, however, the charging rate will always be limited by the maximum power that the source can provide.
Using a different cable will have no impact on your iPad’s ability to retain charge.
Please be assured that the suggested cable is completely suitable - and is designed for the task. As with all cables, some are higher quality than others. Other manufacturers are available elsewhere.
Mophie and Anker are both good cable manufacturers. I suggested the Mophie options as these are marketed by Apple both online and instore.
On the basis of the bad information that you received from Anker, I’d be inclined to vote with my wallet.