Whether it is appropriate or not, the Linux kernel is used in many mission critical and essential services. One could very convincingly argue that the open source Linux kernel is a vital part of mission critical infrastructure found all around the world. An apology, likely written under duress, for an inappropriate research method or for consuming the time of maintainers (either volunteer or paid) does not address the most egregious transgression:
One does not conduct security research or experimentation on mission critical infrastructure without the informed consent of the persons maintaining or responsible for said infrastructure.
The stark difference between white (or even gray) hat and black hack security operations is whether those operations are done with the informed consent of the owners of the system(s) being manipulated. To knowingly and deliberately attempt to manipulate critical infrastructure without the informed consent of the persons responsible for that infrastructure is, in my eyes, clearly unethical black hat hacking. It does not matter how many resources were expended nor does it matter if there was no personal injury or destruction of property. A quiet, relatively uneventful outcome of a black hat operation is nothing more than a dose of good luck.
There are many other circumstances in which this sort of clandestine operation would be obviously unacceptable. Consider if a researcher wished to surreptitiously tamper with vaccine supply chains, automotive or aircraft supply chains, fuel distribution supply chains, chemical manufacturing supply chains, etc. Even if the researcher claimed to have the best of intentions, and had a written plan to carefully back-out or otherwise reverse (or prevent from going to production) the intentionally flawed manipulations or modifications of critical supply chain components, I think very few people would consider the research ethical. There are an enormous number of issues, defects, and mishaps that occur even when a group of people tries their best to implement and maintain critical systems. When a clandestine operator decides to interfere, using whatever justification, then reliability of the system can be expected to suffer.
I do not believe the written apology addresses the underlying problem, which seems to be that the University of Minnesota does not exercise sufficient governance over the researchers in its employ.