There is some confusion around those terms.
The “Secular Humanists”, in their manifesto, declare themselves to be atheists, and Secular Humanism itself a religion.
And even “atheist” is a misnomer for most of these folks. If you don’t believe in pink unicorns, you don’t go around complaining about them and folks that do believe in them.
Most “atheists” these days are “anti-theists”. It’s not that they don’t believe in the Creator and therefore don’t care. They actively seek to remove any vestiges of faith from all aspects of life (and don’t give me the BS that their OK if you practice it in private, I don’t buy it).
Just because they are confused about the term, doesn’t mean I have to be.