There is definitely truth to your theory, I have observed the same in my personal interactions. For example through general conversation I found that a Hong Konger friend who is an ardent critic of the PRC doesn't even know all the provinces in China. The lack of knowledge of Chinese culture and history is not limited to millennials though, under British colonial rule many Hong Kong students have to choose between studying Chinese and Chinese History or a Western foreign language such as French or Spanish and Eurocentric "world" history. The latter combination was held in higher regard by many locals due to the relative rarity, supposed difficulty because it's different, perceived demand in higher status social circles/better job prospects, and mere affiliation with the colonial rulers, exactly as the colonial rulers wanted.
I can't find Supersnoop's post to reply to so I sort of having to hitchhike on yours.
I think he's in danger of overgeneralizing Overseas Chinese by his reference to "pro-democracy" supporters of overseas local-born Chinese millennials.
The ethnic Chinese in South East Asia have a long history of immigration to the region stretching back to 600 years (The time of Zheng He voyages) and still retain their Chinese culture and language. I believe this is also true to some extent in some western countries although to a lesser degree in terms of population. As opposed to most western countries, there is very little recent immigration from China to SE Asia in the modern era.
Perhaps a line should be drawn on the supporters on both sides (in Vancouver, I haven't heard of any demonstration in relation to HK in Asian countries) between those who immigrated from HK for political reasons in the first place and those who immigrated from China and HK for reasons other than political.