Yup, putting on a sweater is a more complex task which requires deliberate planning and coordination, it requires more cognitive control as opposed to tossing your shirt which is a simple reflexive behavior. Sleepwalkers can perform highly coordinated and complex behaviors as it does not happen during REM sleep where your body is paralyzed, but sleepwalking is a distinct phenomenon, plus when you are not sleepwalking, the brain's control over motor functions is very limited.

Not everything is lost though, you can potentially learn it through repetition, you just have to do this in a half-awake state or in a state close to sleep. Do it often enough, and theoretically you may be able to perform the task during non-REM sleep.

I swear I used to be able to switch off my alarm clock while still fully asleep

Must be why there are apps that make you solve a math problem or take a picture of your bath room to turn off the alarm.

It's exactly what I use "alarm clock xtreme" or whatever it's called. Math puzzle, retype or even scan barcode to stop alarm. It has been invaluable, especially as you can change difficulty, I found out even half asleep I could do basic addition etc...