Stack Exchange doesn't have the PM option as of now and pinging them to enter chat doesn't work all the time if they've never visited chat.
What you could do is comment your concerns on the post and ask them to join you in chat in the comment. Arguing isnt condoned anywhere, but if they join you you could have a conversation in chat about it. Then afterward comment the chat conversation to the post if it's appropriate to.
Good luck on this, if you still think the post is incorrect you can downvote accordingly. Flagging would work, but only if the post fit the criteria.