Don't know where you get that from, hot air means less power and less power at the wrong time means oops at best or SH***T otherwise.
Note I said radient heat, you feel it on exposed skin much like a heat lamp, there's not much of a rise in OAT which helicopter performance is figured on. Note also I said you avoid flying in the plume, where the heat of combustion causes a significant rise in OAT which would affect your performance. So unless the flare plume is spilling over the helideck, and I've never seen that before, then my comment still stands that the flare has little affect on the helicopter.