The Skiller can decide whether he wants to give up to 5 minutes free. Its good in order to check if they match. And in case that the 5 minutes free was enough for the whole service the user can tip the Skiller and everyone will be happy

How do you prevent people from abusing this 5 minute grace period?
We don't. its up to the Skiller to give them. If the Skiller notice that the user is not serious he can end the call immediately.
I see. Is the call charged per second or?
It's probably paid per second yeah.
They did say that the first minute is free, where did you see 5 minutes? Maybe I read it wrong

Maybe we all got lost in the whitepapper

I think thats something that someone from the team confirmed recently,about Skiller having option to give up to 5 minutes free.
That's an excellent strategy to get people to try your product and see if it works for them, not having to spend a lot of money/tokens if it's not really what they wanted.