What else besides churches was at the time built using stave and post construction? What they found is not a church per se, but some holes in the ground and some (not much, probably) wood in these holes. Thus a church is a conjecture, unless there is more to it, like fragments of church ornaments, vestments and so on.
You mean like when they find a piece of bone the size of a thumbnail and extrapolate an entire prehistoric human being from it???
I was thinking the same as you, until I read the article..
The gravesites however, would tend to validate the assumption that the building was a church.. If not in the beginning, then soon thereafter..