While it may seem like a flip-flop, the absence of the public scheme does introduce the need for compulsion.

"Individual mandate" is necessary if insurers aren't allowed to discriminate on the basis of prior conditions/risk. If people don't have to insure, many will refuse to do so until they develop an expensive condition at which point they'll seek insurance so that somebody else can pay their bills. You'd be a mug to pay premiums while you're healthy if insurers aren't allowed to refuse you when the time comes. There won't be any insurers if that happens.

It's also true that the best way to keep costs down is to have as many healthy people in the pool as possible. Without individual mandate it won't happen.

If there is a public scheme you don't need individual mandate because everybody effectively is covered if they aren't insured.