Looking at Sisson's site and quoting from it (note I said 28 lb but it should have been 30 lb) -
"All Sisson Kayaks are custom built these days. You can choose the type of footrest, type of construction lay-up and all-up weight. We can advise you on what lay-up would best suit your overall needs. We built our first Kevlar kayak in 1982. It weighed 49lb all up and was used by Paul Caffyn on his Australian trip. In 1985 we went for broke and built a 30lb (14 kg) all up Nordkapp for Paulís Japanese trip. It is our understanding that this kayak is still in use more than a decade later. Our Kevlar experience goes back to 1977. Our early Kevlar projects involved the creation of helicopter fertiliser spreading, spraying and deer recovery equipment. Building the very best Kevlar kayaks was easy after that."
Relating to a standard order -
"Std Weight: 24kg
Kevlar Weight: 22.5kg"
Note also he says multiply kg by 2.2 to get pounds.
That's about all I know about his construction methods but they do seem to last.
Personally I stick to using wood so am not biased by Sissson's use of carbon or kevlar, flexible or not. Wood is stiff, light and strong enough. That is wood without a covering of glass, just epoxy.