Agree with you about Codecademy's assumption/expectation of a layperson's understanding is ambitious but your expectation of what needs to be explained kinda deviates from their aim. Yes, they should've expanded on some concepts but Codecademy should really concentrate on the 'what' and not the 'why' as that can be developed afterwards. Spending most teaching on the 'why' (like most adult learners would do) early on will put off some users off as they do not feel they are 'doing' much and quit/move onto something else.

It is obvious the users' background covers a wide spectrum so Codecademy would have to draw a line somewhere and say here's the pre-requisites prior to starting the intro coding course. They may need to provide this background knowledge via a pre-sessional/foundation/principles course (or pintpoint users where to go if they don't) covering the barebone basics of computer science and computing as a task.

Whether 'the hard way' is the best is really up to the learner - people learn in different styles so whatever gets their attention really. As a suggestion , Codecademy should provide a list of additional resources for eager learners who wants to understand the underlying concepts and/or provide links to other courses if this particular course is not useful for them.