Apple (or any platform) provides a service for people / companies to voluntarily develop software to offer to the users of that platform. What cut they take is an agreement entered into based on people's assessment of the costs and benefits provided, and people are free to decline it and take their efforts elsewhere. Their terms for their platform are not hindering people from competing and offering services on another platform. Each developer is free to make his/her own judgement about whether that commission is tolerable and desired, for the value created.
If the agreement if free and voluntary, Apple could set any price it desires. You, in your own business are free to (and probably generally also try to) set the maximum price you desire. By some similar logic, all apps should charge the same price to be seen as fair. What principle says that one platform's price has to be in line with others? It's what your offering can justify, and what people are willing to pay, with free choice.
The market for Android and Windows app stores has settled on exactly the same rates as iOS apps, which is exactly same rate as game-console platforms that have only one exclusive store. 30%, across the board.
Android alone is sufficient evidence that sideloading doesn't force the dominant app store (Google) to lower its rates to compete.
That's probably the easiest way to discredit this report, their Windows Store numbers are wrong if they're claiming all sales are charged 30%. Microsoft only charge 5% for a sale referred from another website, and only 15% for a sale that originated in the store:
https://blogs.windows.com/windowsdeveloper/2018/05/07/a-new-...
Google seems to already be out in front of the likely ramifications. I got a Google survey last week asking opinions on spinning the Play Store off as a separate non-Google owned company, how much I would pay to use the Play Store, and opinions on competing sideloaded stores (eg Epic Games Store, Amazon)
Apple App Store charges 30%, and it's the only way to publish a native app on iOS, but the Google Play Store charges 30%, too, and Android supports sideloading. The Windows Store and Steam both charge 30%, too, even though it's trivial to download games for Windows from any web site.
It would be reasonable to guess that Apple would exploit its market position to charge higher rates than the Google Play Store or Steam, but that hasn't happened, and it doesn't seem like it's ever going to happen.