Earlier quoted context omitted.
If you hide it you risk being discharged a month, a year, or five years from now when your employer finds out, and then they aren't referenceable. If I were hiring for a security-related job, I'd consider the poster if he not only disclosed the conviction, but also contextualized it on his cover letter the way that he did in the pastebin, and if he provided some character witnesses (teachers, employers).
But (and correct this ignorant european), but doesn't at will employment in the USA mean that they could be fired st any point for any or no reason anyway? And that's if they were never convicted of anything, right?
What you say is essentially correct, but there's enough gray area that companies would not want to fire someone carelessly. Even winning a termination suit is losing, for PR reasons. So most professional jobs give enough severance to move on to your next job, unless it's "for cause", which usually means you did something horrible (but occasionally not).
Lying about a felony conviction is "for cause" and he'd get no severance if that was discovered and the reason (or discovered and made the reason after the fact). There is no dispute about that.
If a company will hire him in spite of the conviction, it's better to disclose. Most likely, he'll start disclosing it in 10+ years once he has a track record and no one cares. If no one will hire him, however, he might need an alternate strategy.