As a rule, if it doesn't say "opposed roll" it doesn't use the opposed roll rules.
Specifically, using Burst requires a casting roll of 4 or better to cast the spell, using the normal, unopposed Trait test rule. Only once the spell is cast do you place the cone template. Once you've placed the cone template, then targets get an Agility roll to avoid the damage, using the original casting roll as the target number.
This procedure is distinct from an opposed roll: the rolls are separated in time, and both can succeed. If it was actually an opposed roll, then they would happen at the same time and only the winner gets what they're aiming to achieve. That is, if Burst used an opposed roll, any target that beat the casting roll would cause the spell to never happen. That would be a strange way to model a "flamethrower" Power, so that's why it doesn't use an opposed roll.