Turns out you can force Siri to go through ChatGPT by treating it like a command line. The ungrammatical query “ChatGPT When does Apple next report quarterly resultschatgpts an accurate answer. I'd grade the answer A-, losing points for not saying "today” (which ChatGPT.app did).
So this means that if you use Siri and want better answers, you need to type “chatgpt" at the beginning of every query.