Can muscle memory and strength be developed by doing guitar scales or drum rudiments while focusing on something else, like watching a movie?
Learning a new concept is much more than being able to repeat (or even reproduce) given information. Learning also encompasses how to take the learned concept and apply it to new scenarios. This type of thinking is called Cognitive Problem Solving and is important for demonstrating how well a given concept is understood.
When I took conducting pedagogy classes as an undergraduate, the professor tested our knowledge of different time signature patterns by giving us a set order to conduct (say 3/4, 4/4, 2/4) while reciting our personal information or while having a candid conversation. Our ability to multi-task while simultaneously performing the exercise illustrated that our brains formed enough neurological connections to be able to allocate the remaining resources to having the conversation.
Now, this type of learning is not always appropriate.
When learning a concept for the first time, it is important that the person give the learning process as much intellectual resource as possible to ensure the most thorough initial understanding possible. This is because neurological connections are being made as the person asks questions and makes correlations in their brain between the new concept and previously learned concepts.
Since multi-tasking divides your attention, you are less likely to form meaningful neurological connections, and the connections that are made will also be mixed in with connections that are created while you're doing the other thing that you were doing. For example, if you're learning how to juggle, but you're also listening to Journey while someone's explaining juggling concepts to you, the information you learned about Journey will be stored with the information you've learned about Journey in your brain.
Since those divisions in your brain would then not be clearly defined or organized, it may take a trigger like Journey to help you remember a specific juggling technique.
Initially, dividing attention reduces the probability of learning, reproducing, and re-contextualizing information. It is only when the person feels confident with the information that they should apply it abstractly to other situations (such as watching a movie.) An old teacher of mine calls this "creative performance practice".
Hope that helps.