I think you both might be talking about the same thing. CBT is much more detailed and comprehensive than "practicing letting go," but in a sense, accepting reality and then letting go (of our maladaptive beliefs and coping behaviors) is at its core. GP may have encountered CBT from an alternative source, and thus doesn't associate it with 'mainstream therapy.' Which I'm not even sure if CBT is prominent enough yet to be considered the main clinical paradigm.