No, they don't need a 'new' client for that. That already exists. Since minecraft is basically open source, it's not really a secret that how to make a client that simply don't require login. (Surely you don't want to say it loud or legal team of MS is going to chase you hard, but lots of client just slip that function in silently)

Besides that.

The minecraft login is basically a license check allow you to join 'legal purchase required' server. And whether you want to enable the enforcement is up to server owner.