"So I'm to assume that unless an artist points out exactly which bits are his/her work then the work is all by others?"

Not at all, but you also shouldn't assume they created everything themselves when no outside credit is given.  If it really matters to you, then just ask the artist. 

"I don't think 'paint and brushes' are really quite the same as DAZ content, ..."

Why not?  They are both objects that you can purchase to help create your art. 

"Users that post their renders in online galleries should describe how their scenes were created." 

Why should online galleries be different?  Online or not, when I visit a gallery, I'm there to enjoy looking at someone else's work and sometimes to get ideas and inspiration.  If I want to specifically know how something was done and/or with what, I have no problem with asking the artist.  Now I know some folks do list much of the creation process in their comments including their mood and the phase of the moon at the time.  Do I want to read about it on every image I view?  Heck no!  I've got better things to do with my too little free time.  Should everyone then be listing all that for the folks who do?  I think, only the folks who feel like it.   Boy!  You want to talk about how things have changed?  Why did posting an image in a gallery become so demanding?  You used to just post an image and folks would come view it and leave a comment if they wanted to.  Now it's like winning at the Oscar's.  You're expected to tell everyone how you made it and thank everyone who helped make it all possible.  Only you don't get a gold statue.  ;-)      

Irene-