Short sale against the box

[johntavv]

In reading 12, they write:
Short sale against the box: investor borrows 100,000 shares and sells them short. Investor is long and short the stock, a riskless position expected to earn the risk-free rate of return.
I see how it is riskless, but how are they earning the risk-free rate of return here?

 

[cpk123]

the sold proceeds are invested in the risk free asset.
and an assumption that the interest rate on the borrowing is not too high - so “almost” the risk free rate is earned.