IMO, you handled the situation perfectly. Covering up for him only undermines your relationship with your kids. Kids are intuitive and know what's going on. Never lie to your kids to protect him and his addiction. What they need is a parent they can trust, and a mother who shows them by example how to deal with unhealthy people and situations, not the fantasy of a father in shining armor that does not exist.
I have spent my entire adult life trying to fill up a hole left in me by a father who essentially didn't give a crap about me. The way my mom always told me he loved me, and covered for him, really effed with my head. I wish she had just been straight with me and told me the truth-- that he was incapable of really loving anyone. Then I could have begun addressing the problem at an early age in therapy. Instead, I learned denial and mistrust from my mother, and still had a dad who didn't love me.
I know your girls are too young to be told their dad is an alcoholic with NPD, but they shouldn't be lied to either. They know he is unreliable and unavailable, just like they know you are reliable and available. If you were late picking them up somewhere, I doubt they would stare anxiously out the window waiting for you because they have confidence that you are there for them. Telling them some BS excuse like his car broke down when he stands them up, is bad for them, bad for you, and bad for your relationship with them.
You can not let a narcissistic alcoholic's comments about your parenting have any effect on you! Consider the source!!!!
Hugs