"Some regularity" is such a wishy-washy phrase though. It's right up there with "ritual purposes" (when archaeologists don't know what a thing was used for in pre-historic times).

Human sacrifice every 9 years is still pretty rare. It's not like it happened every equinox, or on the cycle of the moon, or anything like that.

Also it's difficult to take records from one place and extrapolate them to the rest of the Viking Age, because we simply don't have records confirming wide-spread human sacrifice. We have records of some human sacrifices in some locations. But certainly not in all locations where Vikings lived (nor even in a majority of them), and the places where archaeologists have found evidence of human sacrifice seem to be few and relatively far between.

So I'd argue that the comment "incidental and rare" for the Viking Age is still true (since we don't have evidence to the contrary). Both statements could very well be true too. It could have happened with some regularity in one location, but that could be an outlier.