The terms body and substance must be read in their technical context, not simply in everyday language. Here jawhar is described with reference to that from which another is composed, whereas jism is that which is itself composed of other constituents.
ومعنى الجوهر: ما يتركب عنه غيره، ومعنى الجسم: ما يتركب هو عن غيره
Substance is what another is composed from; body is what is composed from others.
Being a constituent of a composite and being a composite oneself are different relations. The argument excludes both from God. Other philosophical definitions exist, but these are the specific meanings at issue in this passage.