I doubt it's true. I've spent so much time around cricket types and the amount of utter rubbish that gets talked is quite something. Speculation about all kinds of stuff, selections, batting orders, fielding positions, use of bowlers...people make up excuses, invent personal conflicts, repeat things their mate's cousin said about a game...I dunno. Doesn't exactly read true that a guy who's been on a New Zealand contract since about the age of 19, who would be doing very comfortably, would chuck it in unless he got an extra 50k - I don't see that he has the reputation in T20 cricket that Boult has where he could go to any league he wanted around the world for a big payout to be used as leverage.
If we're going on baseless theories one my own is that it's a combination of Australia making Cummins captain, therefore it's now officially okay to have a fast bowler as captain, and Southee being our closest approximation to Bazball in terms of attitude. Stead, Larsen and David White strike me as the types who still look to England and Australia for all their cricketing ways and they've seen those things and are trying to ape them.