This actually seems rational to me. They grossly overpaid for ARM, and selling it on the public market may well end up being another huge and embarrassing write-down. On the private market, though, they had a shot at finding a buyer to whom ARM was worth more than its value as an independent company.

Of course, given the kind of company that ARM is, any such buyer is going to be, almost by definition, an antitrust situation. So maybe it never could get past regulators. But, from a purely self-interested standpoint, it makes sense for them to try.