The second author only ever discussed what happens on the primary Mac. He implies some of the things that happen on secondary devices -- including a second Mac -- but if you read carefully, nothing he says rules out what happened to the first author.
By all means, I agree with you that the first author shouldn't have installed Apple Music on his DAW without being intimately familiar with all the ramifications of doing so, I certainly wouldn't.
But the second author is obviously just an Apple fan, trying to reassure people without actually saying anything. Personally, people like that just make me more wary.