Japanese tend to be less bilingual than most countries so you won't see them online engaging in international forums as much. I don't think they are antisocial. I would say they tend to be more insular or tribal. Meaning they are less likely to reach outside their social groups. Americans for example if they have a gathering with work friends or tennis friends or whatever it's common to invite your spouses or other friends to such gatherings. In Japan, they typically don't mix friend groups like this even when it's just a social activity like drinking.I am comparing people all over the world how I socialize with them from time to time. I see lots of people from all over the world are quite social, especially the West but Japanese don't go online quite often.
I agree with you about the first part but not with the second part for the very reasons that you gave in the first part.I think its important to say that Japanese CULTURE is anti-social as well as anti-social inspiring and not really the people. The culture makes Japanese people feel and act very awkward especially around new people. They are weighed down by cultural rules that just make socializing hella inconvenient and uncomfortable. From specific words that have to be said in certain situations to seasonal obligational gift/card giving to excessive and competitive bowing to the "proper" way to hand someone a business card and the need to even give them one, and other formalities, its just exhausting to try and be social and get all the supposed necessary details right. Any new acquaintance is like "Ah damn, I got to jump through a thousand hoops again." And its worse meeting foreigners cause now they have no playbook to work from at all where they were used to having a playbook for every move. How can you be casual and spontaneous without being creepy when you have never had any practice whatsoever? You can't.
The fact that there isn't (much of) a playbook when meeting a foreigner means that Japanese people can often be more at ease with foreigners than with Japanese