This review of Claire-Louise Bennett's Pond begins: "Pond is an experimental novel that takes place entirely inside the mind of an unnamed protagonist." The review really doesn't discuss any other of the novel's formal qualities that might make it "experimental," so we must conclude that the reviewer does believe what makes it experimental is that it "takes place entirely inside the mind" of its protagonist.
Writers have been writing novels that in whole or in part attempt to narrate their stories by going "inside the mind" of their characters for quite a long time now--at least 100 years. (The reviewer mentions Joyce's Ulysses as an analogue of what Bennett is trying to do in this novel, so even she is obviously aware of this history.) It is hard to understand how a literary strategy with such an ancient lineage (ancient in the history of the novel) could still be called "experimental."
Is narrative realism (stories that can be traced out according to the requirements of Freytag's Triangle) still so thoroughly the default setting for works of fiction that any deviation from it, even one that has been used over and over again, is still perceived as experimental? Is the particular variant of realism on display in Pond--since "psychological realism" is still realism, just not necessarily presented as a linear narrative--the only departure from the imperatives of conventional storytelling that can be counted as experimental? Is this the reason why other writers who attempt other kinds of language and form-based innovation in fiction are hardly ever reviewed in mainstream publications by their favored reviewers?