NO because Nietzsche is using "slave morality" as a designation for those following Christian precepts of surrendering to god, serving the church, suppressing desires and the lie of theological chiasmus (he who is least amongst us is greatest and he who is greatest is the servant of all)
In Nietzsche's philosophy Christians have slave morality, Christianity as an institution has Master Morality and a vested interesting in maintaining that status quo.
Atheists have no part in the predicate.

Nietzche's concepts of master/slave morality do not equate to christianity/atheism.They don't really belong on the same axes.

To use the old cliche. Only if not collecting stamps is a hobby.

I think you're thinking "master" is the opposite of "slave" whereas I would say it is the compliment. How about "free"? The master is more like the controlling deity, not the free thinking independent being unfettered by theistic shackles.

The implication here would seem to be that Christianity and atheism are polar opposites, which of course they are not.

Nietzsche instituted an Ayn Rand brand of Atheism. I chose to be a free thinker. I have my own sense of morality and I try to stay out of trouble. I don't define morality for anybody else. They know right from wrong. Free will. Some people chose wrong.