Jones is contrasting Democritean and Platonist understanding of matter. In atomism matter is the ultimate reality of moving atoms, and space is its polar opposite, the non-being, the void. In Platonism the substratum of the sensible world, the source of brute facts, is "always in the process of becoming and perishing and never really is", devoid of any characteristics, a shapeless "Receptacle" upon which the forms impress. Jones's way of stating it is that physical elements are not "material" but spatial.
There is a more detailed discussion of these issues in Johnson's Three Ancient Meanings of Matter: Democritus, Plato, and Aristotle, that reaches the same conclusion, but phrases it somewhat differently. The physical (sensible) can be said to be "material", but matter, such as it is for Plato, is synonymous with amorphous, "hardly real", but receptive, space:
"The atoms and their quantitative determinations
are in all respects eternal and undergo no generation, corruption, alteration, increase, or diminution. The sole change to which they are subject is that of local motion... Democritean matter cannot function as potentiality for the plain reason that it is permanently and uninterruptedly actual in all the respects in which anything for Democritus ever is
In the case of Plato the answer is radically different. "What is that," he asks, "which always is and has no becoming; and what is that which is always becoming and never is? That which is apprehended by intelligence and reason is always in the same state; but that which is conceived by opinion with the help of sensation and without reason, is always in the process of becoming and perishing and never really is.
If we now turn to the conception of space in Plato we again encounter the sharpest contrast to Democritus. Whereas Democritus had made matter and space ontological opposites, they are identified
by Plato. Space for him is the Receptacle, necessary as in Democritus to account for motion; but it is now also analogized with a Mother and nurse who, being impregnated by the immaterial essences, provides
the very stuff out of which the sensible world of becoming is generated. The Receptacle is at once "hardly real," a postulate that is required by the fact that we say "all existence... must of necessity
be in some place and occupy a space," and at the same time "in some mysterious way partakes of the intelligible" so that although it is devoid of determinate and exclusive form it nevertheless is "duly prepared" to "receive the impress" of any form and to be the substratum of the created world... Democritean space is the ontological reverse of
matter; Platonic space is identical with matter."