Which bit?
> they found one Covid case since Feb or > one Covid case prompted the lockdown
They've gone into lockdown because the man "is deemed to have been infectious since Aug. 12" and "visited a crowded pub on Saturday night [Aug 14th]" and "the probability of it being delta". I should imagine they don't want a superspreader event coming from that when they've been C19-free since February...
In my state in Australia we have only had two very brief lockdowns of several days since the pandemic started and they were to let contact tracers get out ahead of a small outbreak when they were concerned that circumstances were not in their favour.
You lock down at one so that you can limit the spread of the already in the community cases.
Do you still think crazy?