His boats must be really light (5 kg total?), since the largest SeaLect oval hatch (rim and cover, I just weighed one) weighs 1.1 kg. !

About the same as Kayak Sport's (I checked) which I think he was referring to. He must have been exaggerating or installing a lot of hatches.

What use is that?
Sea and bays round the Tasmanian coast.

I don't understand the hatch construction, but it sounds like the 'cover' is a piece of kayak skin attached to the boat with velcro. Is that correct?

Yes. Using one side as the hinge. The goop is flexible enough.