Okay not sure how this works.
Corp unit at range 4 from an AT ST drop a smoke token to get light cover from being shot at. Atst shoots, they have light cover (assuming full unit is in range 1 of said token etc)
Now based on the following rule
A vehicle unit within range 1 of a smoke token does not improve its cover, and units defending against a vehicle unit whose unit leader is within range 1 of a smoke token do not improve their cover.
What happens when either player drops ANOTHER smoke token in range 1 of the AT ST rules as written, the infantry unit no longer improves their cover because the qualifier above is that the vehicle leader is in range of A token, instead of specifying the same token as the defending unit.
So if I'm correct, all a player needs to do to negate a defender's smoke is to provide smoke of their own around their vehicles.