Just want to clarify this, and other future buffs in this event, and how they work. If they have '+' at the front, this means this will be an additive buff alongside everything else.
In your example here, it is stacking additively with all your other Multiply buffs taking it from +340% to +390%.
Any other buffs that you see as part of this event using this language will work the same way. We'll take the feedback on board though and try to make this clearer in the future however.