It's a loan, they'll be charged interest.
Also, £70k for a car isn't exactly "billionaires only" territory. Okay, it's out of the loan-free grasp of the majority of people, but certainly not impossible for a decent % of them to buy with the aid of a loan.
I think the NYT's guy is hoping that his odd interpretation of Moore's Law applies to photovoltaics as well (some of them are based on silicon too, so it must! It's true!). Which would make it viable to have a viable (given El Reg's figures on their efficiencies) solar car in... ooh... 8 years? Say 10 for one that'll get by in an Aberdonian december without having to sacrifice some heating and a CD player for sitting in traffic.