This is almost certainly an intentional leak by the Ravens to a reporter to get their side of the story out without explicitly revealing the private details. It's not some lower level person sharing info without authorization. It's intentional and planned and purposeful. This happens in all industries that have heavy media scrutiny.
This isn't like the Roe v Wade overturn leak from the Supreme Court. There won't be any hunt to find the leaker. It was info given away freely to make a point publicly.
Yeah, there was a lot of outcry over him not signing, and certain media personalities have been painting the team/owner as cheap. Getting out the info that they offered something like 6/290+ helps go against that narrative, even if the details are murkier. These things aren't from "spies" they are intentional.
It also could have been the NFLPA. Benefit to them of leaking is less clear cut though. Maybe to get out there their support for guarantees, or to show that they were helping Lamar with advice, I don't know.
It certainly wasn't anyone from Lamar's side.