ETA: Come to think of it, scalar quantities are seldom the same as vector quantities.
The quantities aren't, but they're just different aspects of energy-momentum, see
wikipedia.
That Wikipedia article supports my point to the detriment of yours.
It takes momentum as a scalar quantity (the magnitude of vector momentum), but it doesn't say energy and momentum are
exactly the same thing. In fact, the article includes several equations from which it is trivial to prove that energy and momentum aren't
exactly the same thing. Here's the first of those equations:
E2 = m2c4 + p2c2
If the energy
E were
exactly the same thing as the scalar momentum
p, then we could substitute one for the other in that equation:
E2 = m2c4 + E2c2
from which it would follow that the energy
E is always the square root of
m2c4/(1-c2), which would mean the cannonball's energy depends only upon its mass. Unless you want to argue that the cannonball's energy is independent of its velocity, its energy can't be
exactly the same as its scalar momentum.
Maybe you were trying to argue that energy is the same as scalar momentum multiplied by the speed of light. If so, we can repeat the above calculation to get this:
E2 = m2c4 + E2
Unless you want to argue that every cannonball has zero mass, that interpretation doesn't work either.
When citing Wikipedia articles that refute your argument is the best you can do, your argument is laughable.
As everyone who knows anything about physics has been trying to tell you: Energy and momentum are related, but they aren't the same thing.