Ok, but the core issue you're intentionally ignoring is that nobody wanders into New Zealand without being vetted by border control. Except for a couple of oddballs who boat in (to a couple specific ports) every single person comes through an international import.
The core issue you and everyone else who makes this islands arguement is intentionally ignoring is that no country has been flooded with covid spreaders through a practical inability to enforce its borders. That Czechia or anywhere else doesn't only have people coming in through a couple specific locations is a policy decision. They could effectively be an island if they wanted to.
The US has tried and failed for 80 years to close its southern border.
Unless you're suggesting the US station ten million soldiers on the border and shoot anyone who tries to enter (hint: this is not politically feasible) closing the US is hopeless.
It doesn't matter "how much" came through. If even a handful of cases come through the border, and you have no internal restrictions, it's exponential spread. You either have 100% hard borders, or there's no point.
Also, both of our largest islands are far larger than Czechia. The South Island/Te Waipounamu is over twice the size of Czechia.