A study reports: smoking increases heart disease risk with hazard ratio $HR = 2.9$ (95% CI: $[2.0, 4.2]$). A sensitivity analysis for unmeasured confounding uses the formula:

$\text{Maximum plausible } HR_{adj} = \frac{HR_{crude}}{1 + \frac{RR_{conf}}{(1 + RR_{conf})}}$

where $RR_{conf}$ is the product of the unmeasured confounder's associations with exposure and outcome. The analysis shows that an unmeasured confounder with $RR_{conf} = 2.5 \times 2.5 = 6.25$ would reduce the estimate to $HR_{adj} \approx 1.0$ (null). What does this imply?