For the specific rule you'd go to page 58 of the BRB;
"Any vehicle weapons of Strength 4 or less (or with no
Strength value) are classified as ‘defensive’ weapons."
Pg 25 of the DE Codex
"Poisoned shooting weapons work in a similar way to poisoned close combat weapons. They do not have a Strength value...and always count as defensive weapons"
Poison weapons don't have a strength value, they wound on a set number, therefore they are defensive weapons when vehicles have them.