There are now two Google Calendar channels at IFTTT. Everything related to the old version is explicitly marked “old.” The new version allows, among other things, selecting a sub-calendar. Before, every applet fed the one cal configured during the give-access-permission IFTTT-gCal connection screen.
This select-cal feature has long been offered in the iOS Calendar channel. Competition, working as desired!
Source: Do more with Google Calendar – IFTTT