I've found the quote I saw -
With those circumstances in mind, NESN's Keagan Stiefel made a prediction on Friday: Anthony won't be up in the majors until late May at the earliest.
"The club would lose a year on his arbitration if the 20-year-old won 2025 American League Rookie of the Year," Stiefel wrote. "It’s not exactly crazy to think Anthony would be in contention if promoted, so the smartest thing to do in their current state is to wait things out — for that reason, and a few more.
"It’s extremely unlikely that he debuts anytime soon, so just keep those thoughts on standby until Anthony falls out of contention for individual awards and back into contention for the call. It’s looking like late May or early June, folks."
And this was the corresponding tweet from Stats.
Like you I can't find anything that suggests there is a hard deadline to fall out of contention for the award, and so maybe Stiefel is making the assumption that being called up that late just automatically leaves you with too much to do. But then, I'm not sure why Stats is quite sure on the 60 game point either. I believe Anthony still has time to stay down to make sure we don't burn a year, but not 60 (well 50 odd now) games...
Weird. It's the kind of rule you'd think would be laid out in very black and white terms to avoid confusion.