You ask: “Who spoke truth? Newton? Einstein? Both? Neither?”
The short answer is that Einstein is more general and contains Newton as a limiting case for low speed. For economists, however, this is not the crucial point. What is much more enlightening is that both shared the same idea of truth.
Newton: “Could all the phaenomena of nature be deduced from only thre [sic] or four general suppositions there might be great reason to allow those suppositions to be true.” (quoted in Westfall, 2008, p. 642)
Einstein on generality: “For this theory [the Theory of Relativity] revealed that it was possible for us, using basic principles very far removed from those of Newton, to do justice to the entire range of the data of experience in a manner even more complete and satisfactory than was possible with Newton's principles.” (1934, p. 166)
Einstein’s fundamental methodological question: “If then it is the case that the axiomatic basis of theoretical physics cannot be an inference from experience, but must be free invention, have we any right to hope that we shall find the correct way?” (1934, p. 167)
Same answer as Newton: “In a certain sense, therefore, I hold it to be true that pure thought is competent to comprehend the real, as the ancients dreamed.” (1934, p. 167)
Same answer as J. S. Mill: “What are the propositions which may reasonably be received without proof? That there must be some such propositions all are agreed, since there cannot be an infinite series of proof, a chain suspended from nothing. But to determine what these propositions are, is the opus magnum of the more recondite mental philosophy.” (2006, p. 746)
Scientists eventually establish the truth (=material and formal consistency). In economics, indeed, all models are wrong. Economists are stuck in the swamp of opinions because they messed up Mill’s opus magnum. Heterodoxy’s task is to get beyond political blather and to spell out the ‘thre or four general suppositions’ knowing from Orthodoxy’s failure already what the false answers are.
Einstein, A. (1934). On the Method of Theoretical Physics. Philosophy of Science, 1(2): 163–169. URL
Mill, J. S. (2006). Principles of Political Economy With Some of Their Applications to Social Philosophy, Vol. 3, Books III-V of Collected Works of John Stuart Mill. Indianapolis: Liberty Fund. URL
Westfall, R. S. (2008). Never at Rest. A Biography of Isaac Newton. Cambridge: Cambridge University Press, 17th edition.
Preceding Two steps towards truth.