They "still have unrevealed Intel Macs" as per the keynote; given that the 2013 iMac is still supported for Big Sur, we almost certainly still have 7 years of support for both architectures in MacOS.
Jobs said the same thing (about new Macs) about PowerPC in 2006. By 2007, Macs were all Intel, and the Intel-only Snow Leopard debuted in 2009. Complete abandonment of Leopard in 2011.