I wonder what you consider valid opinions then.

You're redirecting the conversation. Is this intentional? I didn't claim any which way as to the "validity" of opinion: I agreed with Saagar, saying that the fact of where you live is easy to obscure or lie about on the internet, and opinions that were possibly deceptively broadcasted should be taken with a grain of salt.

Suppose a foreigner posts a video on Youtube, showing that he's on the street in China, speaking about Hong Kong. Would you say that that is a valid opinion?

This is a fun question, because there are four or five layers to it! I'll try to answer it and the rest of your questions the best I can, because I don't think we're that far off from ideological alignment here.

First, I'll start off again by pointing out your usage of "valid opinion" isn't here nor there (all opinions are valid, even if some are deceiving and many are wrong): my statement wasn't on the validity of opinions, but on whether or not the opinions were coming from people who were being honest about where they were from. But I still like this question, so I'm going to answer.

You word this like it's a "gotcha," as if for some reason expats are primarily on the side against CCP (or China, because let's be honest, a lot of the hate directed in the CCP's direction is anti-Chinese xenophobia, despite there being a lot of valid reasons to hate the CCP and detest its actions), but most foreigners who'd be willing to film themselves talking in China and publicly broadcast it are pro-Chinese government. I don't think this is inherently a bad thing, and it's completely natural given that the Chinese government has some absolutely fantastic scholarships and has relatively accepting immigration policies.

But then, Hong Kong isn't China. I wouldn't trust an American living in France to tell me about Venezuela, even if their opinions are "valid" as long as they're representing themselves as an American living in France telling me about Venezuela. I'd trust a foreigner living in Hong Kong to tell me about Hong Kong, I'd trust a foreigner living in China to tell me about whichever part of China they're in (I'm not going to claim someone living in Chongqing could tell me much about Huangshan; people don't accept that China is more like USA than a tiny country like France, but I wouldn't trust a Californian to deliver a good summary of what Wisconsin is like, either).

It then follows that Chinese living in China and Americans living in the USA have opinions that are worth less than anyone living in Hong Kong, which I'd say is true if you think people should have a right to self-determination. Most online opinions claiming to be from Hong Kong online should probably be discounted then, solely because given the volume of people from both countries with an interest in interfering in Hong Kong's affairs (let's be honest, CIA is assuredly interfering too given the obvious benefits of an independent Hong Kong to US intelligence).

Also, if we're so skeptical about all sorts of online comments, do you believe I should consider all HN comments about anything political, to be invalid too because they might be disinformation campaigns?

Again, you're using "invalid" in a sense that I don't think is useful. Nonetheless, anyone claiming to be from anywhere online has a pretty strong chance of not being from there. Any dog can log on. So yeah, definitely discount HN pretty heavily. Even if HN isn't subject to any disinformation campaigns (ignoring that Amazon's been caught paying people to comment on here and some Amazon accounts have been banned for that), it's still primarily well-off people circlejerking about the dangers of economic systems they've only read the Wikipedia articles about; yes, absolutely throw the opinions away. Most importantly, do so when sides not directly involved (meaning not Hong Kong in this case, despite that China is 'directly' involved; China is 'directly' involved in the sense that USA was during Vietnam) have vested interests in making things go their way.