Actually, that's not true.

What causes RSI is mostly centered around tendon and their sheath tension.

For example, if you use a straight keyboard (don't! Ergonomic all the way!) that is raised toward you as opposed to away from you, keeping your hands on that keyboard for multiple hours in a row without ever reaching for the mouse is pretty much guaranteed to give you RSI. Reaching for the mouse will actually bring relief to your tendons.

Another keyboard habit that will put a great deal of stress on your hands and wrists is leaving the Control key at the bottom left of the keyboard. Typing a control key with this configuration will bring great stress on your pinky and other tendons in your hand.

Recommendation: map Control on Caps.