That is a common misunderstanding of how learning curves work. A steep learning curve means your skill increases more rapidly with the invested time. That means the subject is easier or more intuitive.
Does this graph mean what it is supposed to mean for the joke to work? The black line means I learn much quicker with less time investment, i.e. it is easier than all the others.
The timing did fit though :) Btw, i can only recommend their book on the immune system. I learned more by reading that than in my semester of immunology.
Let me guess: Kurzgesagt inspired this meme
Squish that cat