Maybe if you commented on it, liked it or chatted on the livestream. Otherwise I don't see how they know you actually viewed it.
OP of the Reddit submission claims they didn't comment or chat.
But afaik YouTube livestreams with a chat also have a user list for who is present, at least they used to when I last checked.
Isn't there a "chat"-like feature similar to Twitch, where you can see everyone's usernames?
Yes, I checked and in the chat window you can get a list of participants. I'm watching the (non-Ukraine) stream but not in the participants list, so I assume you have to actually say something in chat.