Cross-plane optical links would have a trickier tracking problem.
While there's no explicit mention of same-plane vs cross-plane optical links, I assume that the first time people have a public cross-plane optical link, they will make a big deal out of it. :)
The article also mentions that SpaceX would need to do further study before using laser links between satellites and ground stations-- this kind of optical link would require both more angular tracking and probably atmospheric correction as well.
If they could only do in-plane links, they would have barely any acquisitions per day, because most links would stay up for long periods of time.