Kant insists the perceived thing outside me (on which my awareness of the temporality of things in me depends) is ‘not…the mere representation of a thing outside me.’ (B275) So he means to deny that what Dicker calls ‘objects in space’ are representations, but as Kemp Smith observes, in the first edition Kant had said the opposite: ‘Outer objects (bodies) are mere appearances, and are therefore nothing but a species of my representations, the objects of which are something only through these representations.’ (A370, quoted in A Commentary to Kant’s Critique of Pure Reason, p. 313). Reinforcing this is that the only alternative referent for ‘thing outside me’ would be a thing in itself, but that won’t work here because the Refutation needs the thing outside to be perceived, i.e., be an object of experience.
Berkeley could accept the so-called Refutation of Idealism in B275, unless the perceived permanent thing Kant is appealing to is the thing in itself. Kant says Kant's view is an ‘empirical realism’ that distinguishes it from something he calls ‘idealism,’ but he hasn’t refuted anything just by claiming a label for himself. In Kant's theory, to be empirical is to be the product of a mind’s synthesizing, or some component of a mental synthesizer; a body is ‘real’ only in the way being a product or component of mental synthesizing lets a thing be real—which is compatible with Berkeley’s account of body. Berkeley never said anything that conflicts with Kant about the empirical reality of body.
I’m aware your question mentioned Descartes, not Berkeley, but part of the point is to challenge Kant’s positioning himself in opposition to both of them, whereas the real contrast is between Descartes on one side and Berkeley/Kant on the other. Kant distorts language when he calls Descartes an idealist; something even stranger is going on when he continues to call his own view ‘idealism’ after inserting a refutation of idealism. You’re right to be unsure what the point of his proof is.