Unbounded divisibility can mean that wherever one marks a boundary in a magnitude, a further subdivision remains possible. It does not by itself mean the magnitude has already been assembled from infinitely many independent substances.
Example: a line may be halved repeatedly; performing or imagining further halvings is not the same as discovering a pre-existing physical particle at every potential boundary.
This is a central response: indefinite potential division is not interchangeable with an actually constituted infinite aggregate of parts.