He isn't pivoting going public with the opinion he'd apparently held privately.
Here's a member within the NYC-DSA arguing against it: https://www.socialistmajority.com/theagitator/chi-osse-endorsement
Starting an independent socialist political party I don't think changes the discussion whether Chi Ossé would contend well against Jeffries, and the idea that time effort and money would be better spent working towards a seat in the NY State Assembly.