AFAIK there's a difference between the TTS options you can run directly and the TTS options that are actually available to the system as a voice for other programs to hook into. Don't quote me on that though.
Either way it likely wouldn't work though, as one of the devs said this on the Discord a while back:
Yeah I couldn't figure out how to get that working with Construct 2/NWJS
Even if I could force it to speak using Javascript commands, I wouldn't be able to 'watch' it to see when the synthesizer is finished speaking, or pause and resume the speech, like I can on PC and Mac
It's something I'd like to revisit post-launch, but for now it's just gonna be captioned 'robot voice'