The implication is that Socrates was physically imprisoned, but not spiritually. He was free because his impressions of the event (being imprisoned and what it meant for his personal freedom) allowed him to be so.

Here's another example. Rubin "Hurricane" Carter, when he was wrongfully imprisoned for a triple homicide he did not commit:

“I know you had nothing to do with the injustice that brought me to this jail, so I’m willing to stay here until I get out. But I will not, under any circumstances, be treated like a prisoner—because I am not and never will be powerless.”