I've never heard of someone doing this, and it's not a bad hack since it's apparently free. My main question would be the effectiveness of an ad like this vs a simple message to the people directly.
As a recruiter (which I was in a different life), would I even notice an ad like this? I'm not sure, but I'd definitely notice a message in my inbox saying "Hey Dave, I found your profile doing some research on $COMPANY ..."
Since he's only targeting 15 companies doing a quick search on LinkedIn for HR/recruiting types at those firms and sending a message like that could probably be done in under an hour, and I'd think it might be a bit more effective than the ad.
It's a clever idea, but I'd question how effective it might be vs a direct message.