So I found this a while ago. The Heli has its own invisible range in which it fires. This range is about as big as an Ice Tower's and moves with it. This range can be used by the sub's Advanced Intel, which gives you an effectively infinite range to fire in.
I actully noticed this on bloons monkey city mobile a week ago because of a monkey teams event. Its pretty interesting but im not sure if there is really a great use for it, unless you have random trio in BTD Battles Mobile.