I’m sure it’s awful for normal folk like us but in his case (and again this is uninformed speculation), where he’s essentially the fall guy for a scandal that was supporting one of the world’s largest economies (and a large component of several others), it’s not that hard to imagine his experience being just a bit different than what a normal inmate should expect.
He also likely knows a lot about how many of his superiors were in the loop on this, so I assume he was well compensated for his inconvenience.
He was in "Offener Vollzug" (~open prison). That means he sleeps in a normal prison cell (that usually looks like [1], so not all that bad) in a regular prison, gets breakfeast, leaves prison to go to work, then goes straight back to prison to participate in the prison's evening activities (sport, recreational, educational, etc). He might get vacation (from staying in prison), and can visit his family on weekends.
It's not that unusual in Germany, at any time about 16% of prisoners are in "offener Vollzug", and it is a great tool to reintegrate prisoners into society. It is limited to first offenders with no flight risk and no risk that they use their time out of prison to do crime.
Of course no matter how useful of a tool it is generally, it does make the prison sentences of some well known people look like a bit of a joke.
Corruption is a real problem in Germany and auto makers have a lot of influence on politicians, so the corporate and the political apparatus are interested in hiding these crimes, because a large part of our economy depends on them.
He also likely knows a lot about how many of his superiors were in the loop on this, so I assume he was well compensated for his inconvenience.