This is a good point. We need to think much more deeply about what it means to be a non-creedal religion.
"The problem here isn't humanism vs. theism for theist Unitarian Universalists -- it's the non-creedal nature of Unitarian Universalism"
The first thing I want to say is that there is more than one possible understanding of non-creedalism. The Disciples of Christ are a non-creedal church, they say here:
"Freedom of belief. Disciples are called together around one essential of faith: belief in Jesus Christ as Lord and Savior. Persons are free to follow their consciences guided by the Bible, the Holy Spirit study and prayer, and are expected to extend that freedom to others."
Quakers are also non-creedal and say here:
Quakers have no set creed or dogma - that means we do not have any declared statements which you have to believe to be a Quaker. There are, however, some commonly held views which unite us. One accepted view is that there is that of God (or the spirit or divine) in all people and that each human being is of unique worth. This shared belief leads Quakers to value all people and to oppose anything that harms or threatens them.
So there is more than one way to be creedless.
What I want to say is that we are a creedless religion. And that word "religion" has to have some meaning. And it has to be something more than ethics and politics.
My answer to this question is that we gather around the Sacred - "that transcending mystery and wonder" but accept that it is mysterious. It cannot be contained in a creed or any static form.
Humanism, as I understand it, refuses to engage with the Sacred. So I don't see how it fits. If it does fit within this understanding, then I don't really understand what it is.