That's a good question McGrath, and I don't know enough to give a right answer. It looks to me like the protocol for serving in a foreign army is extremely vague, and no one has been very forthcoming about what it was exactly that Lindh did.
Twenty years does seem a long time - but chances are he won't serve all of it.
Mister allowed that Lindh's story is probably the most intriguing thing so far to arise from this conflict. I can't say as I disagree with him. And That is probably why Steve Earle wrote the song.