In the context of software and content, iOS is a Closed System as users are restricted to access apps and content at Apple discretion. Unlike for instance the macOs and Internet. Users of these systems, access and supply apps and content at their discretion.
Not long ago, the Internet was in danger not to be an Open System. But regulations came to make sure Internet Service Providers could not impose restrictions on their users regarding access and service to content and services / apps and countries got together to establish a common standard that everyone would enforce. That has proven to be the right formula at many, many, many levels, including security. It became a platform for wealth and progress at an unimaginable scale.
Today's Apple itself is a child of openness as it benefited from it, like everyone else. Paradoxically, they seam to have difficulties to take on the same level of responsibility for it that others have taken. For instance if some businesses had their way when the Internet was being built, it would be Apple paying them to have the ability to connect their devices (revenue share maybe?). Even after the Internet was built, the same kind of companies tried to argue that they should be able shape traffic and their discretion, turning traffic shaping into a business, using the same kind of arguments Apple is using. So again an attempt to close it at the infrastructure level.
Now, it is highly debatable that very large computing platforms of software and devices such as iOS, macOS, Windows, Android and others, supporting simply one Web Browser is an enough take on responsibility when others have to taken on so much more, including having their business models regulated. Some might consider the argument that supporting just the Web Browser while pursuing typical practices of closed systems beside the included Web Browser, at scale, is simply hacking the openness of the Internet by shaping its synergies to their profit through the devices people use. Some, might argue that the effects of such hack are being seen in "miracle" financial results.
The DMA is just set of regulations, demanding that very large scale computing systems, aka Gatekeepers, technically de facto core components of the Internet amongst others, aka Gatekeepers, assume their responsibility in such openness like others have done. It is just a matter of Gatekeepers deciding which approach will give them the most profits in the future. Scale down to the point of not being Gatekeepers or scale ahead as much as they can, sell as much devices and platform licenses for profit as they can and accept sharing the responsibility as other adults in the game have.
Cheers.