> which is trivially disprovable, not least because the pipelines were not transporting any gas and hadn't for a while.

This doesn't disprove anything. Energy prices could still have surged for multiple reasons, e.g. because countries might have worried about similar attacks on other, active pipelines and might have become more eager to boost their reserves. We'd have to look at some actual stats, but even then it might be hard to tell apart any effects from the utter market chaos at the time.