No, because it hasn't. I think you misread it.
"The work done by friction on the car is related to the initial kinetic energy of the car. The work-energy relationship is often related by the equation
KEi + PEi + Wext = KEf + PEf "
Which is different from what you said.
Then they point out that the PE is the same throughout, assuming the car is going along level ground.
"Since the potential energy of the car is the same in the initial state (before braking) as the final state (after stopping),"
And then they cancel all the irrelevant and null terms
" each term can be canceled from the above equation. And since the car is finally stopped, the KEf term in the equation is zero."
So that KEi + Wext = 0 though they don't bother to explain, just substitute straight in
"Thus, the equation becomes 0.5*m*v2 + F*d*cos(180) = 0."
Basically KE = Work done by skiddingIn the absence of external gain or loss of energy, such as a ball rolling (or sliding) along a frictionless surface in the absence of air resistance, then KE + PE = KE + PE , so if it stops, it must have gained PE.
But here the lost KE went into the friction.