This is a valid cellar
Besides being valid, it also has a perfect cellar score. It works because the chiseled blocks are counted as soil (the base material) and by chiseling them at a diagonal there is no direct view to the outside when constrained to an axis. Besides being useful for access, this can be used to transport items in and out of a cellar, you could even setup your quern in there. One thing of note, vertical placement of a cellar does not affect cellar score, it will always use the sea level temperature of your world when checking outside temperature.