Is there a true Dhamma?
To my mind the answer is both yes and no.
Yes. I think that the Buddha distinguished between Dhamma and adhamma. Some things do lead to Nibbana and others lead only to more samsara. The Dhamma seems to include such ancient formulations as the Three Marks, The Eightfold Path, The Four Noble Truths (or Realities), the Brahmaviharas, and the like, and especially the realization of these teachings. Adhamma explicitly includes such things as the Atman, self-glorification, greed, attachment, doctrines of permanence, a Creator, and everything that leads away from the Path to Nibbana.
At the same time, I think that one of the points of AN VIII, 53 (=8.169) is that a great many
things may be compatible with the teachings and may support one’s journey to enlightenment. The Path is actually quite broad in that the Buddha seems to say that it includes anything (consistent with the basic teachings I imagine) that gives rise to or enhances your nonattachment, renunciation, simplicity, contentment, etc.
Therefore, if you have a relatively firm foundation in the teachings and practices that come from the Buddha, as well as those which legitimately developed out of these early on, then I think that you may recognize Dhamma in many, different places, not just one. (Your clarity may protect you from mixing Dhamma with adhamma in some of the more unusual Dhamma-recognizing contexts.) One example of learning from another place: I learned from one teacher that problems and adversity are always an opportunity to practice, to develop compassion, nonattachment, insight, and forbearance, and to become aware of how my mind creates its own suffering, partly through the way in which it interprets and embellishes upon experience. I learned from him that with this attitude we may train ourselves to see difficult circumstances as a great help, rather than a hindrance, and incorporate them into the Path. I learned from him that every moment
is a teacher and a time to practice, if we remain aware. (This brief summary does not do his teaching justice.) Now this teacher is not a teacher of Theravada, although he certainly embraces Dhamma and rejects adhamma, as defined above. Should I reject this true, helpful Dhammic advice, because I happened to find it in a non-Theravada package? I don’t think so. True wisdom is still wisdom wherever you may find it. I think I’d be crazy not to learn from someone who has cultivated wholesome states to a degree far beyond my own. And I don’t think that confusion or the intermixing of incompatible elements need be the result, at least not for me. I have always desired to understand as many points of view as I can. That is who I am. I still practice Theravada.
I think that Dhamma is one body of truth and many in its forms of expression and practice.
I am a bit sleepy. I hope this didn't come out too garbled.