vsync?

No, there's no vsync. Realistically I don't think the update rate is such that you would notice tearing with animations anyway.

Could you make the documentation a little bit more nooby friendly, explicit on these terms and what the tradeoffs are?

Maybe you could suggest some changes? There's a lot of information to get across and if it's too verbose nobody will read it.

So the mystery remains, why doesn't 'terminal' work with the doublebuffered mode

It's not a mystery - there are two buffers. Text is only written to one buffer, not both at the same time. If it wrote text and then did a flip each time for an update, you'd get the display flickering each time, showing half the text.