Link to article? I find it very hard to believe Mac is retiring and his cap hit is being removed, because he would have to forfeit his remaining salary.
TSN claims that MacArthur has now been put on LTIR (he was previously on regular IR). If true, Capfriendly hasn't been updated to reflect that. This is not the same as Mac retiring and the Sens being rid of his cap hit.
I think people get confused when there is talk about MacArthur "retiring". Just like Savard or Pronger, MacArthur is probably "retired" in principal as in he is too hurt to ever play again and has put his playing days behind him, but on paper (cap purposes) he is still a member of the Senators roster. He can't fully retire and have his cap hit removed from the Senators books, or else he would not get his salary owed to him.
Unless MacArthur fully retires (which he won't for the reason above) his cap hit will be on the Senators books for the rest of his contract. The Senators may opt at one point (or may have already if TSN is right) to put MacArthur on long term injury reserve which allows them to spend over the cap by however much MacArthur's contract would put them over. This isn't the same as removing his cap hit.
It also has to be considered that the Senators are a budget team, so MacArthur on LTIR doesn't necessarily mean the Senators get that money back to spend elsewhere. Cap and salary are two different elements. It's been rumoured MacArthur's contract is 80 percent insured, which if true means the Senators would be getting back all but roughly 1M of his salary.