It's not always a smart play to completely identify the speaker of a poem as the poet him/herself. But with Stevens, it's hard not to. After all, when you write a poem called "Of Modern Poetry," and the whole thing is a lecture about what poetry should be like, who else are we supposed to think the speaker is? The speaker definitely sounds like Stevens himself.
The one thing we should do to clarify this, though, is say that the speaker is Stevens at the time he wrote this poem. Let's not forget that people can (and probably should) change their beliefs, opinions, and emotions over time. So if Stevens is the speaker of this poem (and it's reasonable to say he is), he's speaking from a specific moment in his life when he felt and believed the things he says in "Of Modern Poetry."