The Egyptians and other cultures really did use twenty-sided dice as evidenced by studies and artifacts, such as the one at the Met.
For example, "A Demotic Inscribed Icosahedron from Dakhleh Oasis" by Martina Minas-Nerpel
published in the The Journal of Egyptian Archaeology in 2007 (Vol. 93, pp. 137-148), describes "a unique icosahedron found at Qaret el-Muzzawaqa in the 1980s and now housed in the New Valley Museum at Kharga" (pictured above). Mina-Nerpel writes,
It probably dates to the first century ad. In contrast to other icosahedra
known from Graeco-Roman Egypt, this one is not inscribed with Greek or Latin letters or numbers, but with 20 Egyptian divine names in Demotic, thus adapting Egyptian concepts to a Greek form [...] The polyhedron was presumably used in an oracular procedure intended to establish which deity would provide help to the petitioner.
She goes on to cite examples of Egyptian icosahedra including: three "made of steatite or faience, inscribed with Greek letters" in Cairo and Paris museums, "four polyhedra from Egypt with Greek letters made of faience, serpentine, steatite, or calcite" housed in the British Museum, "a faience icosahedron inscribed with Greek letters" sold on the art market, and a rock crystal icosahedron "inscribed with Latin letters and Roman
numerals" at the Louvre.
These are described (or "published" in other words) in the literature and regarded as authentic by scholars and collectors.
For more information you can begin with a chapter by W.J. Tait called "Dicing with the Gods" in Egyptian Religion, edited by W. Clarysse, A. Schoors, and H. Willems (Leuven: Peeters, 1998).
It is worth mentioning that Platonic solids (of which icosahedron is one) were also known to the Greeks, at least as far back as 300 BC. Euclid attributes the discovery of the icosahedron to Theaetetus. (See "The Discovery of Regular Solids" by William C. Waterhouse in Archive for History of Exact Sciences, Vol. 9, No. 3, 30.XII.1972).