If Flaps 30 is used on a 737-800, that setting is more likely used because the aircraft flies like a drunken pig on ice skates at Flaps 40 with any cross/tailwind
Most probably if you use the same speed (even bit higher due to wind),
for both flap settings...
and it is common for speed to rise just before or during flare with any tailwind..
nowhere does it state that the engines must be spooled up
You are most probably right. But why should have it been stated at all ? How can you be stabilized at 500' or 1000' with IDLE power ? Unles you are in a glider..