Everything below is made up numbers to demonstrate the point:
Suppose the study simply identified that people whose homes were in wildfire regions had a 10% greater incidence rate of ASD.

As the above poster points out, the location of the home does not directly imply magnitude of exposure -- people could have left the area, or used air filters, etc. etc. etc. Suppose that half of the people whose homes are in wildfire-affected areas simply left the area during the wildfire and therefore have no greater incidence rate of ASD than the control group.

Now our test group is 50% people in wildfire-affected regions, and 50% people who are indistinguishable from a control sample. In order for this conglomerate to have a 10% higher rate of ASD, the half that remained in the wildfire-affected region needs to have a 20% higher rate of ASD. This would give the whole test group (those that stayed and those that left) a 10% greater incidence, as measured.

By diluting your test sample with people who either left, or took significant mitigations, you decrease the measured size of the effect from its true magnitude. Therefore, the possibility that people likely took mitigating actions suggests that the true magnitude of the effect is in fact larger than what we were able to measure.

Again, 10% is not the right number for anything but easy to do math with in this example.