Why should I have to ask for a raise from my existing manager before asking for a raise from every other manager in town?

If my manager wanted that, it would have been in the contract [that I have never had as an "at will" employee], and they would have had to pay something to me as compensation.

If I have no contract, I owe my employer nothing. The loyalty I give them is equal to the loyalty they promised me: zero. They explicitly told me I could be fired at any time for any reason from an enumerated list ten pages long, or for no reason at all, if the whim merely struck them one day.

As an employee, that is a big "fuck you" to me from the management, and they should be happy that I even let them know I might quit if they don't do something soon to prevent that, instead of just calling my manager up to say I won't be working there today, as another company offered me a better deal.

Fine but then how to you back up your claims that you actually are worth a higher salary?

I suppose it primes the discussion with "does my employer even see me as valuable? Perhaps more valuable than I am being paid for?"

If the answer to that is no, no amount of backup will convince them otherwise. Then you get into the question of "why am I working here" and it forces you to leave anyway.

Somehow, all paths lead to leaving. And I think this is what the original article is getting at - if you want more money, your best bet is to leave.