This is a little idealistic. I agree with both employer (manager) and employee advice in some areas:

  A good boss starts preparing for you to leave the day 
  you start your job. [...] I let them know that if 
  they’re getting the urge to leave, I can help them line 
  up their next job.

I started doing this as a manager, and it's great. You have to initiate the conversation, but it makes your relationship that much more transparent. You can see them relax. And as a bonus, this is a proxy for, "Talk to me about other issues you're having." Complaining about a shitty internal process is nothing compared to talking about leaving.

  Tell your boss you’ll be leaving at the end of your 
  current project, or a specific task that’s on your 
  plate. [...] I’d aim to give your boss about a month’s 
  notice.

Idealistic, but I've never met an employer who's that flexible on start dates. Pretty much everyone wants you to start AS SOON AS POSSIBLE. It's hard enough to push for two weeks plus a week or two downtime, let alone double that.

The author (Mike Monteiro) does cover this as something employers should want to see, but it just doesn't happen.