There's an entire career path in international diplomacy dealing with the vexedness of most nations' "One China" policies but,
- for almost all intents and purposes, China is essentially the People's Republic of China or its government.
- In marketing or int'l travel, it'll usually be explicitly Mainland China plus Hainan.
The adjective Chinese, on the other hand, can mean a ton of separate things:
of or related to Mainland China (大陆);
of or related to its government (中华人民共和國政府);
of or related to Greater China (中國), inclusive of the SARs and Taiwan;
of or related to the citizens of China (中國人), inclusive of Uyghurs, Tibetans, &c.;
of or related to the Han Chinese (汉族人) specifically;
of or related to the written Chinese language (中文), intelligible in various dialects;
of or related to Mandarin Chinese (普通話) specifically.
In B2B contexts, you're also going to see a distinction between foreign companies operating in the Chinese market (whatever the boundaries of that are) versus the local Chinese companies.
In your two bullet points, you're right that it's badly expressed marketing jargon. Taken most charitably, "Chinese social media marketing advice" is advice on marketing on PRC (and Chinese-language) social media platforms like Weibo, Weixin, and Bilibili and "advice on social media marketing in China" is advice on adjusting general social media marketing to deal with PRC consumers and regulators. There's not a lot of daylight between those two categories, but there's some.
That's not really what the glosses suggest they mean, though. The actual people you're dealing with are using the first category to describe "common mistakes in the Chinese social media market" and the second to describe "tips for business leaders to get ahead in the Chinese social media market". That's not at all what the headers are actually talking about. It's just badly done.