Is charging a royalty for using the iOS SDK "gatekeeping"? I don't see why that's the case. Is Epic Games "gatekeeping" Unreal Engine by demanding 5% royalty to use their engine? Why should it be any different for Apple to demand royalty for using their SDK?
Even if EU magically made a law that explicitly banned royalties for operating system SDKs, that won't be the end of the story. Such measures could be construed as anti-free trade (given how it disproportionately affects US companies compared to EU companies), and will probably lead to a trade war and/or WTO arbitration between the EU and the US.
Where they'd get into trouble is by trying to turn these things into ways to move the 30% revenue cut. Just moving the 30% revenue from "rights to be on the one and only store" to "rights to write apps against the device with the one and only SDK" is quite clearly a different motive and forced cost model than having a 5% SDK fee where the developer is free to choose a different SDK.
I.e. "but others charge for an SDK" doesn't allow you to then change your SDK cost model to exactly match what was deemed illegally anticompetitive forced market pricing. It just allows you to charge for your SDK in a competitive way, nothing more.
$99/year for the development kit will probably be seen as reasonable enough for it to be allowed, though they may have to accept allowing 3rd party alternative solutions to publish things to these other app stores or similar. The fixed price of the development kit isn't really the point when talking about the 30% commission though. That's about revenue from consumer purchases and trying to evade that via moving the pricing to the dev kit wholly then requiring the dev kit will not fly just because the DMA doesn't explicitly reference doing that being an example of a bad gatekeeping action.