With respect to education, that limit is definitely intentional for Marx, and the same goes for other forms of so-called "human capital" like job experience and connections.

They definitely help you make money, but they don't have the special attributes that make capital so desireable--you can't sell it to turn it into another form of capital, and crucially you can't employ others to use it to make money on your behalf while capturing the surplus value of their labor.