Agriculture is not the only thing humans do or settle for. Swamps are historically heavily inhabited by and sometimes even purposefully extended by humans specifically due to endemic biodiversity and ease of hunting and gathering. Also, sometimes because they offer fertile soils or refuge from other humans, and the waterways a strong source of orientation, food, and an artery for transport, trade and taxation. Examples off the top of my head: Bangladesh, Holland, Venice, Hanoi, Bangkok, Shanghai, Hangzhou, Angkor, Amazon, Sepik River of PNG...
Virtually all your examples are seaside marshes, not low-lying inland swamps. But your example of Angkor is a good example of a low-lying inland swampy area and, while for a period of history it was a productive, heavily populated area it then experienced a population crash which has extended through to the present day.
I’d add Tenochtitlan which is Mexico now. It was a swamp surrounded by mountains 500 years ago. Densely populated.
So let's look purely at Hanoi, Hangzhou and Angkor as the subset that matches your requested scope. I still think it's fair to point out that they are all enormously successful areas.