Kilograms and joules have been interchangable (well, they've had a defined relationship) for a long time though, you can calculate work in a completely Newtonia

Kilograms and joules have been interchangable (well, they've had a defined relationship) for a long time though, you can calculate work in a completely Newtonian context and see that the energy requires is linearly related to the mass and quadratically to velocity (like with a spring). That the direct conversion from mass to energy follows the same shape isn't really surprising, it sort of has to. That said, the joule was only explicitly defined as kgm/s^2 in 1946 (or 1935), after Einstein and nuclear physics.