Without implying any bad will about the author of this post, who is a recruiter, recruiters are incentivized to close deals, not to get you the best possible package. It is possible that this background would convince a recruiter that advice which is against your interests is still a good idea, because if he adopts advice which will predictably results in employees underpricing themselves, his conversion rate from interviews to paychecks will improve, possibly dramatically. If his placements are routinely leaving $10k on the table, that only costs him $3k per placement, which might on a subconscious level either a) not be motivational or b) might be a totally rational decision because of the increased volume of placements he gets by advising people to take any reasonable deal.

My father is a real estate agent, and real estate agents have demonstrable blind spots (seriously, academic literature exists on this) for negotiation when negotiating for clients versus when selling their own homes. When they're selling their own home, five hours of extra work might add $5,000 to the sales price, so of course they go the extra mile. When they're selling a client's home, five hours of extra work might add $5,000 to the sales price but their commission check only goes up by $300, so they'll generally opt to tell you "That's a really fair offer. You should take it. Can we close this today? I have a young couple who I'd really like to meet with now about buying a brownstone, and the expected value of my time with them is way higher than the marginal value of my time with you. Oh whoops did I just say that out loud?"