The video makes the assumption of a homogeneous workforce across age groups in terms of productivity. The reality is different.
As stated previously most of China's elderly lives in rural areas and small towns. Many just transitioned out of subsistence farming and doesn't participate as much in Chinas economic circulation like that of the younger more educated and more productive population. The education for that cohort is quite low, with most having just elementary education. The largest cohort after the seniors has median education of middle school. When these people retire China's economy would have transitioned to have less opportunities for these people as there would be different skills match. Retirement cost of living isn't as expensive relatively speaking compared to developed nations.
There's a reason we see China's space agency engineers being mostly in their 20s and 30s, these educated people to fill those positions didn't exist previously in such numbers. Lead engineers for many civil projects like bridge building were also of similar age. That large educated cohort just didn't exist in decades past. The retiring cohort is not an exact match to the younger cohort as we would assume in a developed nation.
The large cohort that the video talked about supplied much of the cheap labor in factories and construction. This dividend will decrease but not totally cease as there's still many low income households. Anti poverty campaign seeks to end absolute poverty but not eliminate the low income population. Production of many low end tradable goods that can't be automated will likely move overseas, especially to ASEAN nations.
The video also referenced the decline of Japan's share of global manufacturing percentage. Although true, this highlights criticisms made by Chinese officials and members of the PLA years ago, despite massive growth in China's share of global manufacturing it still lacked ability to produce many core technologies that will act as chokepoints. You can produce all the textiles, toys, assembly (which counts as manufacturing) in the world but it wont support a modern economy by itself. Developed nations faced a decline in share of global manufacturing but they still hold the chokepoints of a modern society. The less educated elderly population in China aren't the ones producing those key technologies by in large.
I see China's economy continue to progress for the next few decades as it climbs the value chain and securing core technologies and offshoring of low end production. The demographic issue will become more impactful once the average Chinese education level approaches that of developed nations. China should seek to abandon limits to urban fertility since there are also many who choose to not have kids at all that needs to be balanced out.
China should have a state sponsored version of udemy that gives all courses online for free... open domain all knowledge and learning materials to lower the barriers to knowledge and deflate the higher education bubble. China would do well to democratize and subsidize education at all levels for everyone always accessible everywhere..
For starters how about online semiconductor courses to get the next generation of students and researchers on the right path... sure beats wasting time playing Dyson Sphere Program or the mandatory English classes