Whenever Dharma-body is referred to as “the true self,” that’s a provisional figure of speech, not the ultimate truth.
While all conditioned things are impermanent, that doesn’t refer to the unconditioned. The Dharmakaya, like Nirvana, is unconditioned, and the two are closely related to each other.
From the beginning of Buddhism, it’s been understood that one takes refuge in the Dharma-body of a Buddha, rather than in his gross physical form:
Enough, Vakkali! What is there to see in this vile body? He who sees Dhamma, Vakkali, sees me; he who sees me sees Dhamma. Truly seeing Dhamma, one sees me; seeing me one sees Dhamma.
Vakkali Sutta: Vakkali
He whose faith in the Tathagata is settled, rooted, established, solid, unshakeable by any ascetic or Brahmin, any deva or mara or Brahma or anyone in the world, can truly say: “I am a true son of Blessed Lord, born of his mouth, born of Dhamma, created by Dhamma, an heir of Dhamma.” Why is that? Because, Vasettha, this designates the Tathagata: “The Body of Dhamma”…
http://www.palicanon.org/en/sutta-pitaka/transcribed-suttas/majjhima-nikaya/142-mn-88-bhitika-sutta-the-cloak.html#calibre_link-893
When Pure Land Buddhists, through the Nembutsu, take refuge in Amida Buddha, they are taking refuge in the Dharma-body, rather than in a literal historical person who attained Buddhahood galaxies away, eons before the Big Bang.
Shinran Shonin referred to Amida Buddha as “Dharmakaya-as-upaya,” making the point that Amida, the Pure Land, and the Nembutsu are a skillful device (upaya) for Ultimate Truth to make itself known and complete its work of leading all beings to enlightenment.