Going to take a guess it's their own, https://wondercraft.ai/.

In another comment [0], it's mentioned that the TTS engine is ElevenLabs' [1]

[0] https://news.ycombinator.com/item?id=35832886

[1] https://elevenlabs.io/

It's ElevenLabs [1], according to OP's earlier comments.

[1] https://beta.elevenlabs.io/

according to link's description it's Wondercraft https://app.wondercraft.ai/

Elevenlabs! Those guys are amazing!