I don't know if this is the worst thing a job-seeker can do (I have seen several "don'ts" in my career), but this is excellent advice. Employers still hire people. Your resume will get you the interview, but you as a confident and informed candidate, along with the right personal and cultural fit, will get you the job.
Here’s what I know: employers don’t hire the neediest or most desperate candidate. They don’t have faith that a needy, desperate person will be able to solve whatever problem they’re hiring someone to solve. That’s why the worst thing a job-seeker can do is to beg for the job, in words or actions. When you show up to a job interview full of ideas and happy to meet a new person, you make a real impression. When you show up ready to bend and flex and be whoever you think the interviewer wants you to be, you make no impression. A great deal of the recruiting process in many organizations these days is mechanical, but people still hire people. They hire people they trust. Your resume won’t get you a job, but your solidity at a job interview will.