If the environment was significantly more varied in health impact between twin comparisons than expected, then the correlations they found under estimate the genetic component.
Some randomness is part of the signal being studied, and some is undesired measurement noise to be controlled for. And it is only the latter that is beneficial to be carefully removed or otherwise controlled for.
If the environment was significantly more varied in health impact between twin comparisons than expected, then the correlations they found under estimate the genetic component.
Noise weakens correlation. Removing noise strengthens correlations.
Some randomness is part of the signal being studied, and some is undesired measurement noise to be controlled for. And it is only the latter that is beneficial to be carefully removed or otherwise controlled for.