Usually what people mean by "splat book" these days, and what I assume is meant here, is "book with primarily player rules in it." Not fluff, not DM stuff. This is not what WotC is doing these days, any book has to pull quadruple duty.
What the term originally meant was a hyper-focused book on a particular topic aimed at very specific players as part of a series: "Complete Book of Elves", "Complete Book of Bards, "Complete Book of " with the "" being a fill-in-the-blank that people would call a "splat" like a squished bug. That sort of book ain't happening, because it's bad business.