Both the 70W-75 oils I've used, Redline and Penrite cost from NZ$40-50 per quart or litre and I'd bet the dealer's oil is even pricier. But Hyundai are not forcing us to do the oil change unlike they are with the blue coolant, it's more a matter of us owners wanting to decrease the possibility of having to replace the gear reducer out of warranty simply due to them wanting to save $3 per car. An Ioniq owner here had his reducer fail on his 5-yr old car at 70,000 km and had to pay for labour, several thousand $, with apparently only our consumer laws prevented him from being billed for the gearbox as well. I'd expect the same scenario to play out if mine failed in a few years. I'd happily pay for the new oil and magnetic plug just for not having to leave my car in the dealer's parking lot, nevermind tolerate the stress of getting major work done.
Most EVs by far use ATF in their gear reducers because that has the correct weight and gear tooth extreme-pressure rating (GL-4). Hyundai could have done the same but perhaps didn't because their 70W GL-4 light gear oil is stocked for use in the dual-clutch automatics used in the Kona ICE and many other models. There's no technical reason that I can see why we could not use ATF intended for conventional automatics as a substitute, however there's no advantage to that either, other than potentially cost.
As for the magnet, as I mentioned to recent queries on FB, installing one of any strength is infinitely better than none. Two is just twice times infinity, but that's still infinity. The drain plugs used on my small 4x4 on each of four gear casings have ceramic magnets, relatively weak in today's terms but they keep the oil clean. This is from the manual 5-speed gearbox after 40,000 km.
Just one note about out-of-warranty failures, a 2018 Kona owner here with nearly 150,000 km just had the charge port actuator fail and had to pay all costs, just over NZ$2100. We all know that this is a not an uncommon failure and almost certainly due to a design defect yet Hyundai accepted no responsibility. One owner in the UK said this issue was considered a recall item by his dealer.
