It likely has nothing to do with AoEIV. You might want to join them tomorrow though.
They are broadcasting on their twitch channel a map with background audio from WWII radio, bombings and sounds of machine guns. Possibly a CoH remaster or DLC/expansion, who knows.
As far as I can recall, some time after AoEIV’s announcement in 2017 Relic published vacancies for an unannounced project - besides AoEIV. That might be it.