I believe you are correct. The “bonus” 30 the US gets when it’s attacked early is to compensate for not having extra money while “at war during collect income”, for example:
The US is attacked turn 2, gets 30 IPCs immediately so they have that to spend when it becomes their turn. If this were NOT the case, then the US would only have their initial 30 IPCs to spend during “war time” which wouldn’t make sense.
This is how I understand it.