Unless he dies, gets a pardon, commutation, or sentence reduction first, the minimum he can serve under existing law is ~21 years based on a 25 year sentence and maximum good conduct time.
SBF may serve as little as 12.5 years, if he gets all of the jailhouse credit available to him," Mitchell Epner, a former federal prosecutor, told CNN.
Federal prisoners generally can earn up to 54 days of time credit a year for good behavior, which could result in an approximately 15% reduction.
Since 2018, however, nonviolent federal inmates can reduce their sentence by as much as 50% under prison reform legislation known as the First Step Act.
Are you saying the article is wrong, or did you maybe not consider this possibility?
That's not an automatic reduction (or even as close to it as good conduct time), as it requires successful participation in specified activities, but it is generally available except for a defined list of mostly violent offenses.