>From: "Eliezer S. Yudkowsky" <email@example.com>
If it were true it would be odd, but it isn't.
Einstein came up with the (1905) The Theory of Special Relativity first, followed by the (1915) General Theory of Relativity, so for a start he had a decade of knowledge and data to build on.
If you meant the Special Theory, it still is not the case. He knew that c=c in any reference frame.
Unlike all previous workers, he took that fact seriously. As soon as take that fact seriously, you can derive the entire TOSR. Einstein did this by imagining travelling on a photon, holding c constant in other (now rapidly translating) reference frames. This immediately gives us time dilation. That, perforce, gives us mass and length dilation (if an object with a given velocity translates at a different rate, its length has changed. If with a given energy input it accelerates more slowly, then its mass has changed). That is the whole theory.
He was brilliant, but hardly working in a vacuum.