Yes, this app uses OpenAI's business API, which, unlike the public model, does not use your messages and content for training. You can read more about OpenAI's Business privacy policy here.
Furthermore, Solid Apps doesn't store anything of what you write or the responses you receive and has no access to any of your data. Your data is kept on your device and synced via your personal iCloud.
No, this app doesn't require an accunt.
No, this app doesn't have any tracking, ads or anything to sell your data.
This app doesn't collect any identifiable information.
This app offers an optional speech to text function, which relies on the built in technologies on your device. The audio data is not captured by Solid Apps or sent to any third party providers. On newer devices, the voice data doesn't even go to Apple's servers. This technology is the same used by any other dictation feature on your device.