Scientism taken literally is the statement that the only meaningful statements are "scientific" ones . Ergo, anything beyond the scope of scientism "isn't meaningful". Ironically, this very approach isn't scientific.
If one can make a metaphysical claim, that is meaningful (has meaning), then this is a problem for the above worldview, because scientism would claim that they aren't meaningful.
This is because you can't build a metaphysics on science alone, you can build an epistemology, but good luck getting an ontology. I don't actually think that DawahFilms is treating scientism as an ontology.
His premises do attempt to state that on scientism, only claims with reference to the scientism worldview are meaningful/coherent:
If all conceptions of reality are derived from the natural world, then they must be made up of coherent experiences.
...
No number of coherent experiences can direct one to consider the possibility of something meaningful beyond said experiences.
Just to clarify what I mean by epistemology and ontology, ontology: what we know, epistemology: how we know what we know. Ontologies run into questions regarding the nature of being, questions of the form "why" (I.e. Why does God necessarily exist?) , whilst scientism would only deal with the "how".
So yeah, scientism is an epistemology as opposed to an ontology such as naturalism, but it still suffers from a big drawback in that the validation it so desperately seeks is external to its own mission statement.