See, I'm sure they are basing this on some data modeling. But I'm close to certain those models would've also been based on some assumptions that the low restrictions would've been 'effectively' put in place. They haven't - case in point, we've just let in 1000s of people from a European hotspot city into the country for a fecking football match today, allowing them to mingle and be in close contact with another 60k people. This kind of shit throws models off.
I'm sure if they had implemented a lot of the low levels (i.e. banned all travel in and out of the high risk countries, like Italy etc) there would've been a later optimum time to put in place harsh restrictions. But they haven't done, or whatever they have done hasn't been as effective. But as we're seeing in Italy, as soon as the total infected individuals gets past a 1000, the thing explodes because you've got so many infectious carriers out there.