That they can line up with it if you're facing the right way at the right time does not guarantee that they must have to do with the moon. Get in the right position and any point in the ruin can be lined up with the moon.
That central causeway coming from the larger motte/hill thing is a pretty obvious feature and the fact that it lines up with the moon seems important.
I just don't see any reason to believe they would make it utterly impossible in solo mode. But it's definitely not a bug. For one, it's way too consistent. No one has reported going into solo and finding a different set of obelisks activated for example. It's always the same ones. No one in solo has ever gotten any others to ever activate even by accident yet.
No, I agree that it must surely be possible in solo, but I think it's something people are doing in open accidentally that we aren't easily able to repeat intentionally. There must be some sort of clue to this, but we haven't really found it. We may just be looking in the wrong way perhaps. For example, I'm still convinced that the beacons just feel far too significant to simply ignore. Given that in open we have people tearing all over the place, dropping artifacts everywhere, pulling stupid crap, shooting each other for no reason except that they can, and so on and so on, it's entirely possible that in open people just keep accidentally doing it.
I agree that it seems unlikely that it's impossible in solo. That really wouldn't fit the designed ethos of the game with the three modes being equal etc.
As for whether or not it's a bug, there are things that suggest to me that it isn't working correctly, mainly the fact that the obelisks don't relight in solo and here seems to be no way to switch them back on. This could easily be linked to other obelisks not turning on when the first one turns off.