This is patently false. Smartcar does not require additional hardware. Quoting from their product page:
> Our API works with the embedded telematics module built into most new vehicles. No need for aftermarket hardware like OBD2 dongles.
As it stands now, the documentation is very much misleading as Smartcar makes it sound as if you can use this on any vehicle. Look again at sahaskatta's response, he/she made it sound as if you just have to read the docs to 'unlock your car with an app'. But in reality, you'll also need to have a brand new and very specific car if you want to do this.
Sahaskatta also asked for 'any feedback I have'. I gave that and now I'm being downvoted for my criticism...
My 2014 Chevy gave me 2 years of their mobile app which could unlock/lock the car via cellular for free. I know the car still has a data connection available because if I play songs via bluetooth it pulls in album art that doesn't exist on my phone via Gracenote. I can also hit the OnStar button and talk to someone via the car and I don't pay for that.
That Chevy lock/unlock API may not be a public API but there is nothing stopping someone from reversing it. Or just signing up to use it: https://developer.gm.com/vehicle-apis
Most modern cars offer the same features.
AFAIK the is not a single car manufacturer (not even Tesla) that has any API available. Most cars don't have internet connectivity anyway.
Disclosure: I do NOT work for Smartcar, but have build multiple backends for P2P carsharing and bike rental platforms.