[Merriam-Webster](https://www.merriam-webster.com/dictionary/learn) shows the different subsenses of **learn** relevant here nicely:

>  _learn_ [transitive verb]: 1a(1): to gain knowledge or understanding
> of or skill in by 
> - (i) study, 
> - (ii) instruction, or 
> - (iii) experience 
> 
> [last classification labelling, i/ii/iii, mine].

In spite of M-W's ordering here, I'd say that the default subsense is the 'assimilate knowledge, understanding, wisdom ... often not by design' subsense (ie iii) rather than the 'set oneself to learn by study and practice' (i) subsense. So, with this subsense not highlighting application, design, the intensifier 'hard' makes no sense.

- Work / try / practise / play / study / think ... hard.

But _not_ the non-applicational 

- *Understand / dream / appreciate / pick up / realise / discover / **learn** [default sense] ... hard.

So I'd say you're on the right track ... the complication is that 'learn' is very polysemous.