You can see the headlines though “Apple skirts interoperability law by deprecating API after only one year”. Maintaining a public API is a cost usually only taken in because it has a benefit to the company.
Apple is not required to develop or maintain any feature against their will. The DMA is not written like that, it is much more objective and industry-agnostic.
The EU demands that features implemented in the OS to be used by Apple accessories must be made equally available to competing accessory vendors.
Certifying devices to make sure they're safe for users, like "this cable is certified as compatible, it won't set your iphone on fire", seems fine.
Requiring your app to be "certified by apple to sell ebooks", and then only granting that certificate to Apple Books, not the Kindle app, that seems anti-competitive.
Especially when the user finds out the reason.
If the industry wants better open standards, they the participants should develop those standards and build devices that implement those standards.
What Apple does outside of such standards is nobody else's business - as long as they correctly implement and support the industry standards.