You're chastising him for his "attitude"; what attitude? He's correct and he didn't display any attitude other than simple honesty.
The reason people say it's exhausting to "be nice" constantly is specifically because of people like you. You've made up your mind on something that's completely outlandish and unfair, but you're going to attack him for being "unkind" because he doesn't agree with you.
Stop acting like that and maybe more of these "blunt" people would stop being so damned blunt to you.
There is so much projection here I'm not even sure where to start.
Lets say I'm working with retail worker who has been trained to be as difficult as possible to stop my RMA attempt for something I bought that was broken. I can see things from her point of view and understand her paycheck comes from her financial masters who want her to deny my RMA, but that infringes on my return rights. Now I express this as clearly as I can. Guess what, now I'm an "asshole" because $big_co wants to be able to casually rip people off.
I pretty much got into a yelling match about a serious engine fault on my wife's warrantied car a few years ago. Its only after I was particularly nasty did they relent and do the work. Why does the guy making $12 an hour behind the counter want to deny me my entitled service? Because Jeep told him to limit this kind of very expensive work.
This is so much more complex than you make it. Niceness and kindness do not resolve the fundamental conflict we are having. If anything, it allows the kind person to be taken advantage of. If I wasn't at the dealer with my wife they would have steamrolled her the same way other mechanics have steamrolled her.
The world is a nasty place and how we handle it is situation dependent. Want me to be nice to you? Treat me with some goddamn respect and don't try to rip me off. Because if you don't, you'll deal with a lot of "assholes." "Assholes" who are just working schmucks trying to get through the day without being ripped off.
I think the "kind" people of HN need to stop calling everyone who disagrees with them or have different motivators "assholes." Disagreement, argument, and negotiation are fundamentals in life. Retail staff need to learn to handle that. Its a huge entitlement to be this snotty retail person who hates all his customers. The lady who wants to return the broken food processor isn't a "bitch" because she got you off your freemium game for a minute and she's not a "mindless breeder" because she fed her crying baby on your counter. Or "stuck up" because she had to answer the pediatricians call on her iphone while you were processing the return.
Think of communication as having two layers, an emotional one and a factual one. In your car example, the factual layer remains the same, "You are going to fix my car." But you can either combine that with an emotional layer of "And I am fucking pissed off with you right now because you fucking don't do your job, you useless twit", or you could layer it with "I understand that your job is difficult and you're doing the best you can, I don't want to make things difficult for you, but the contract clearly says that the car is under warranty, the car is clearly broken, and I'm not leaving until it's fixed."