No, that poster doesn't know what they are talking about. That said, sentencing is very discretionary so a judge will commonly balance out your total sentence with how much time you've already spent in jail.
I took that to mean that while you are in jail you burn down one to one days which is not as fast as the bonus or "gain time" you would get in actual prison.
He may or may not be correct and my guess is it depends on where you at since state and local laws will be applied but... As for knowing what he is talking about my guess is many prisoners are experts in the system of trying to reduce their stays in prison.
As the other poster actually has first-hand experience of being incarcerated, your dismissive tone in response to some loose phrasing comes off as both nasty and ridiculous.
Source, was a CO in Texas, also: http://law.justia.com/codes/texas/2005/gv/004.00.000498.00.h...
If you follow the link, you'll see that participating in work or educational programs can improve one's good time earning class, allowing an inmate to earn an earlier release date. In contrast to the other poster, Texas requires good time for time served in county jails. Also notable, educational opportunities have been severely curtailed in TDCJ over the last decade.