Looks like this was bumped up to mach 28. Минобороны опубликовало новые данные о скорости "Авангарда" - РИА Новости, 22.12.2020 (ria.ru)
"It accelerates to Mach 28, follows from the "Main results of the activities of the Armed Forces of the Russian Federation in 2012-2020", which are at the...