No, Bonhoeffer was not a theological liberal!
The problem, of course, is that nobody could possibly mistake the author of Nachfolge for a member of the ELCA or the PCUSA or the UCC.
Yes, he was a student of Barth. And yes, he played the higher critical game. But he also took the Faith seriously.
In fact, as Gene Veith observes, nobody could make that mistake about anyone who took sin as seriously as Bonhoeffer did.