As Andy says that piece was "opinion"....
The typical case is X% of the fixed part of first year pay, AKA "contingency" recruitment. There is typically a cap.
Then there is "retained" where a fixed fee is negotiated, more senior appointments work like that.
In house recruiters do a large % of entry level recruitment, they do this because they are cheap and the work consists of applying a given set of prejudices about what school you went to, followed by simplistic buzzword matching.
In other words, is using headhunters a negative thing for negotiating the offer?
Usually not, one has to understand the utility function of the 3 different people who might negotiate.
1: In house:
Their job is to put candidates who have been filtered in front of the hiring manager, they don't really do negotiation at all. They get paid for this, not for placing people.
2: You:
If you are dealing directly with the hiring manager then you have to balance risking not getting the job with getting the money.
3: HH:
A HH only gets paid if you get the job, so there is more pestering and for a given job you are one of a small set that he might place, so will try rather harder to get you in, trying to get both sides to move their numbers together.