Can I use Siri voices with text to speech in my app with a synthesizer?

Yes — Apple has described Siri voices as part of Siri itself, but not as a general text-to-speech synthesizer API you can use directly in your app.

What the WWDC sessions do say:

• In Bring your app to Siri, Apple says Siri is getting a more natural voice and better language understanding.
• In Keynote, Apple also shows that Siri’s voice can be customized and made more expressive.

But in the context you provided, there’s no indication that developers can use Siri’s voices with their own text-to-speech synthesizer in-app. The speech-related developer APIs shown here are about speech-to-text:

• SpeechAnalyzer / SpeechTranscriber for transcribing audio into text
• Siri integration via App Intents / App Schemas

So the safe answer is:

I don’t know of any WWDC announcement that lets you use Siri voices directly in your app for text-to-speech with a synthesizer. The sessions here focus on Siri improvements and speech-to-text APIs, not exposing Siri voices as a developer TTS voice.

If you want, I can also help distinguish between Siri integration, speech-to-text, and text-to-speech based on these sessions.