That's a ridiculous claim. 2015 shipments of 288.7 million units (Gartner) may be down but they're still huge: that's 791,000 per day. And sales of Windows tablets and 2-in-1s are growing very rapidly.
Are tablet sales "largely irrelevant" because they're lower than PC sales (206.8 million, says IDC), and even if you include tablet PCs running Windows, declining even faster? https://www.idc.com/getdoc.jsp?containerId=prUS40990116