Instead, you quit and go to work next door for $120k, while one of their guys quits his $90k job and comes to work at your old employer for $120k.
The key is that in this case, you quit. If you are actually fired for some reason, then you will have a lot of explaining to do which could dramatically reduce your market value. Even if you were fired for some stupid reason, like racial discrimination, that's a lot of explaining you have to do that other candidates do not.
That's a good point. I believe the original comment was being a bit loose with terminology, and not really making a distinction between being fired, being encouraged to quit, being made redundant, etc. The fact that he also mentioned a letter of resignation points that way too.
You're correct that there is a huge difference between being let go with good references as part of a reorg of corporate strategy (or whatever_, and being fired for sexually assaulting the cleaning staff. The former is little impediment to finding a new job, while the latter is. (Well, unless you can successfully threaten your old HR department with a wrongful dismissal suits so that they will lie and cover for you. And oh yes, that happens[1].)
Context does matter, and that's worth pointing out.
[1]: Frequently, too. A short-sighted analysis says that you have two goals when you discover a bad employee: 1) Get them OUT of your organization before they do something that might make you liable and 2) Make sure they don't sue you. A glowing letter of recommendation is cheap, and can help achieve both goals. Often a small bribe - sorry, severance package - can be tossed in as well to sweeten the deal. Of course, that will come around to bite you when you end up hiring some other companies ambulatory menace on the strength of a glowing-but-false letter of recommendation. sigh
The key is that in this case, you quit. If you are actually fired for some reason, then you will have a lot of explaining to do which could dramatically reduce your market value. Even if you were fired for some stupid reason, like racial discrimination, that's a lot of explaining you have to do that other candidates do not.