What Zack Short said was that Betts was very interested in staying, just wanted to get market value (exactly as Mookie himself said), and that the Sox simply failed to get there. In other words, they lowballed him. Short also appeared to confirm the final offer was a little short of $300 million, which once again supports what Mookie said.
Here's a link to the Zack Short thread.
https://talksox.com/forums/topic/89091-zack-scott-explaining-the-mookie-trade-and-a-lot-more-article-from-over-the-monster/