If by novel you mean a long work of prose fiction, it most certainly is a novel. If, on the other hand, a novel is a long work of fiction with a central through-line that unifies the whole, then perhaps you need some other kind of generic marker for Rachman's book. One thinks of other works of fiction that collect shorter pieces with some common theme: James Joyce's Dubliners for example, Sherwood Anderson's Winesburg, Ohio. These are collections with even subtler connections than Rachman's that have been sometimes characterized as novels, although most often they have been treated as short story collections. On the other hand, a large scale work like John Dos Passos' U.S.A. that not only tells multiple stories, often contains stories that have few, if any, connections — but also includes a number of other narrative elements, and is still usually thought of as a novel.
The novel as a genre has always been difficult to define. From its beginnings, wherever they might be marked, there have always been works that have defied classification for one reason or another: Gulliver's Travels, most of the fiction of Daniel Defoe, Rablais — the list could go on and on. Almost from its very inception, the novel has been a form that has given reign to experimentation of one sort or another, and perhaps this has been its greatest strength. Experimentation can allow the form to meet the needs and tastes of new generations of readers. A generation with an attention span accustomed to the sound bite, flash fiction, and the 10-minute play may well find they prefer their novels chopped into shorter pieces that can be digested in parts.
Whatever you call it, The Imperfectionists is well wrought piece of fiction deserving of all the accolades it has received. Whether it is a harbinger for the future of fiction remains to be seen.