Seems to me an all-too-convient rationale to force devs to use the proprietary Metal framework directly.
The article omitted the critical detail about which private API was used. I have received one of those letters before and they should contain those details. The fact that the article omitted that information makes it sound like they want to start a controversy instead of admitting a library developer probably made an easily remedied mistake. They were hoping for low-information-high-opinion internet audience to drive traffic.
Same reason they refused to provide cross platform Vulkan to begin with. And it's not a good reason obviously.