Yes, they're on topic here. The current policy as stated in the help center and close reason does not make an exception, and there's not enough reason to change that policy. So for example:
Core gameplay (how to play the actual game):
Meta-gameplay (how to play, outside the actual game)
Meta-game (about the game, outside of actually playing):
Off-topic exceptions, possibly better on SuperUser or Arqade:
First of all, we will generally be able to provide good answers. Even if the questions are specific to the computer game, we are likely to have users who play it. In fact, we may even be a better place than Arqade: the leap from playing a board game to playing the computer version is not large, while not all PC/console gamers are going to end up playing computerized board games. Indeed, historically we've done pretty well with these kinds of questions, from the examples I was able to find.
It's also just confusing for some questions about a computerized board game to be okay, and some not. It's not great for us to define our scope in such a way that it's easy for users to make the wrong choice, or worse, easy for us to end up debating whether a question is specific to the computer version or not. For example, a recent MTG rules question turned out to be an MTGO bug - it turns out it is specific to the computer version, but this is still definitely a great place to ask it.
This also means doing a better job of helping users. Whether someone's playing the physical or the computer version of a game, we want our site to be a good place to get helpful answers, and maybe even encourage people's interest in board games.
And finally, at this point there just aren't enough of these questions to worry about. As of September 2014, Arqade had all of two questions tagged [mtg-online], and though we don't have a separate tag, with a little searching I only found maybe 6 MTGO questions. There are other computerized board game questions out there, but I don't think they're too numerous either.
So I don't think it's worth making an explicit change to our policy: it's a fairly reasonable policy, and it's not causing any problems.