In a joint communique issued after the visit, China "reiterated its respect for the independence, sovereignty and territorial integrity of Cambodia, and reaffirmed its support to the Cambodian people in selecting a development path which suits their national conditions".
From the specific perspective, the construction of Kunming-Bangkok Road, China-Laos Railway, China-Thailand Railway, Long Jiang Industrial Park in Vietnam, Vientiane Saysettha Development Zone of Laos, Sihanoukville Special Economy Zone in Cambodia, among others, are underway.
Despite the show of unity between the Mekong leaders, campaign group International Rivers said there were major concerns about the future of the river, with 20 dams planned on the upper Mekong in addition to eight already built.
Environmental groups warn the blockages pose a grave threat to fish habitats by disrupting migrations and the flow of key nutrients and sediment - not to mention displacing tens of thousands of people with flooding.
Communities in the lower Mekong countries have reported depleted fish stocks in recent years and are blaming the dams.
Experts say it is too early to draw full conclusions about the impact given a lack of baseline data and the complex nature of the river's ecosystem. China is Cambodia's biggest source of aid, investment and imports.
Cambodia, as a member country of groups like the Greater Mekong Subregion, plays "a major role in these regional and subregional organisations, and is therefore important in China's strategic interest", said Dr Deth Sok Udom, a Phnom Penh-based associate professor of worldwide relations.
That allows Beijing to keep "undermining habitats and millions of livelihoods downstream".
With control over the headwaters of the river - known in China as the Lancang - Beijing can dam its section while the impacts are felt downstream.
Established in 2015, the LMC had made significant progress after foreign ministers meeting in Dali, China, last month approved the first batch of 132 projects, while leaders yesterday considered a second batch of 214 projects, Hun Sen said.
Suiting the trend of regional economic integration, the Lancang-Mekong Cooperation mechanism has shown strong vitality since its birth. Cooperation on trade, transport tourism and environmental protection was also vital.
The forum is seen as a rival to the Mekong River Commission, which has existed for more than 60 years but excludes China and Myanmar.
Firms and state agencies from Thailand, Vietnam and Laos also stand to gain from their investments in the hydropower projects. Another deal involved Chinese financing for a new airport.
That makes the Mekong's dependents, such as fisherman Sles Hiet, an afterthought.