I have planted the whole lot at the same time and there is no issue with it.
You have to bear in mind that potato foliage gets damaged by frost, so if you plant everything now, you could be rushing to earth up or fleece if we get a cold snap.
I think the logic is that you want the early potatoes as soon as you can, so you plant early and faff a bit over them. The second earlies are a follow on crop, so you plant those a bit later to stagger your harvest. If you don't dig them because the first earlies are still going, they keep growing and turn into an early main crop, but that is not the end of the world. The mains you want for autumn and to store, so the suggested planting times reflect that.
In a cold or wet spring, that can all go to pot and that is when I end up rushing them all into the ground together