You say negligible, but what you mean is negligible on a scale of the CO2 emissions we are used to. Human body CO2 emission is just as negligible on that scale.
Ill do some quick maths: According to the first source on google, an average human burns 3kcal/min walking and 5kcal/min biking at 15km/h, which is about 3x as fast as walking. Considering that, we arrive at a difference of 1.33kcal/min by walking instead of biking. Estimates suggest 1.3g CO2-equivalents per kcal for average consumption (much lower for vegans), so thats 1.73g CO2/min. 100kg CO2 for bike production would take 57,800min of walking, thats almost 963h. An average bike will probably be used more so I guess you are right and bikes are indeed more efficient than walking :D