Also yes it is cardio, or it wouldn’t be good exercise. It’s not HIIT or running, but it consumes energy and calories.
Cardio is a shorthand for cardiovascular, if you do normal paced walking it doesn’t challenge your cardiovascular system therefore it’s not cardio.
If the calories burned are greater than those I can consume to maintain my body weight (currently about 3000kcal/day) then yes it will make me lose weight.
you said lose muscle, which it doesn’t do, in your scenario you are losing muscle because of insufficient calorie intake, not because of “doing cardio”, there is no extra effect from walking that would have you lose muscle other than burning some calories, which is what “losing muscle” means
you can just eat a handful of almonds and you will replenish all that you walked away.