You are correct in your statement.

I should highlight, however, that the answer provided in the case is not necessarily correct. There are a lot of things that are being erroneously disregarded:

• Not considering time-value of money, it is at break even
• Considering that you do a big investment upfront and get smaller ammounts of money in the following 5 years, i.e., considering time value of money, it is negative
• Company faces a lot of risks: that the energy price may be different, that they may drive less than 15000kms, or that they will charge less at home. Of course, variation can happen in the opposite direction and be positive for them. They are not being remunerated for this risk.

But, there are two very relevant things that SHOULD BE CONSIDERED (and these comments help you get an offer):

• We need to know the impact on our car sales. It seems indiferent to do this by itself, but if it drives car sales, then it may be a very good idea. How many more cars will we sell if we offer this?  If for example we have a 5% margin on these cars, each additional car we sell will represent 5%*35k€=1.75k€… that's good money.
    
• Clients may decide to keep on their contract after 5 years… meaning that every time that happens, the client has a relevant profit (of ~60€/year).

Taking these two points in consideration, there is room to argue that this initiative is likely to be very positive.

But let me put this the other way. The case objective is to improve car sales. Disregarding an initiative that is at breakeven without considering its impact on the case objective (increase car sales) should be considered WRONG and reflect negatively on the candidate.
 

And  let me add that getting the math right is good. But what makes you pass the interview round is not the math, but the discussion over the consequencies of the math analysis.