According to the MLBPA website, the salary proposal of $1.5M is a Major League minimum, not a 40 man minimum. ESPN reports that proposed minimum would start at $1.5M for 2027, with annual increases to $2.2M in 2031.
MLBPA Makes Opening Proposals to Benefit All Players and Build Upon Industry Momentum
MLBPA makes 1st proposal in collective bargaining negotiations - ESPN