The claim that Diggs was released is actually incorrect. He was waived.
Per the league rules:
From the start of the season until the trade deadline, only players who have 3 or fewer accrued seasons pass through waivers.
After the trade deadline and until the end of the Super Bowl, ALL players who are on the active roster must pass through waivers if their contract with their current team is terminated.
So, Diggs, like Darius Slay, is subject to the waiver process. The team claiming him is responsible for the remaining contract minus any amortized bonus money that was already paid out.