In the second bullet under "Quick Take" on the page you linked to, it says: "Einstein's famous 1905 relativity paper is valid only for low velocities", which is false. Einstein's derivation of the Lorentz transformation in that paper is valid for any relative velocity (less than ##c##, of course). His derivation of ##E = mc^2## in that paper, of course, is not valid for any velocity, but that's because that formula itself is only valid for zero velocity--see next paragraph.
It then says "in six further attempts he never succeeded in producing a universal derivation of ##E = mc^2##". I'm not sure what "six further attempts" he's talking about, but since the equation ##E = mc^2## is only valid at rest (zero velocity) to begin with, of course Einstein, or anyone else, could never produce a "universal derivation" of the formula. So this statement strikes me as highly disingenous.