News Flash: Militaries whose governments may not want war nevertheless plan for war. We have plans for invading Belgium, updated annually, for example. Not because the USA wants to invade Belgium, but because we want to be ready to do so if we need to.

Making the US agitated serves very little purpose, other than escalate tensions. You don't build and fly sophisticated spy rigs hanging from balloons without a plan, and China is known for long-term planning. It's not ridiculous notion to suggest that an otherwise nonsensical surveillance platform might have a use in a post-war scenario with the USA.