okay here's how i see it:
if you realize there is no self then you are no more different than a floating rock in space is to the universe. a rock floating in space is seen as just another part of the universe unless there is a sentient being to look and label it and decide it's an individual thing. only sentient entities are seen as things with "self", so if one realizes no self, they lose identity and become non dual ideas just like a rock in space with no sentient observer.
or from another perspective:
our bodies consist of a bunch of organs and microscopic cells that all work together yet we consider ourselves only one entity or "self". so we see our bodies as a single non dual entity when considered only in relation to itself, it is one thing. imagine if each organ was sentient, we would consider ourselves many selves and our bodies would be very dual. the same is true for everything, we see one galaxy we live in and then even bigger, one universe, and so on until we have to just give up and say everything is contained within ever expanding infinity. without the illusion of "I" there is no logical reason to consider anything as more than one, and since we can't define the universe in even that sense, non duality becomes just non, an ineffable idea that cannot be posited in any language other than direct dhamma realization.
i also consider imaging a universe with zero sentient beings. there would be no duality at all! it would just be one big thing. only by imagining a separate self can we create a non dual reality.this sutta
just makes things worse lol! but i see some correlation between some zen ideas and theravada.
i don't know that there are any zen teachings not in the tipitaka in some way or another.