hi, I'm Alex at Terrastruct and we made this. we just launched D2 last week, where this site was brought up plenty, so I didn't expect this to come up again. happy to answer q's if any.
(we're working on a playground so that this site's examples can be runnable!)
i'll also use this opportunity to plug that this next release of D2 is going to be packed with features/improvements/fixes that came from feedback received on HN from the launch: