And honestly, can one truly benefit that much from watching a few seconds earlier than something happens?
You can benefit from it if you are fast enough and if the delay truly is 6-8 seconds for example. If you are ready, you can in theory submit a bet in 5-6 seconds. Having said that and by looking at at the second video that OP submitted, I do not believe that the difference between a point on the stream and the bookie changing the score is 6 seconds or more.
And what sort of live bet does it have to be to truly make a difference and ensure a win?
I don't know the rules and betting options for table tennis. But for tennis for example, you could see on the stream that a player has gained a 40:0 or 40:15 advantage which puts him in a decent position to win the next game. And you could bet on that before the bookie lowers the odds or removes them altogether. Maybe there are bets on who wins the next point, etc.
Could OP maybe tell us more about this?
Like, placing a bet that a goal will be scored when you just saw it happen and hoping the winnings will be credited?
Possibly, yes. In theory though, sports bookies aren't idiots. They have considered each and every scenario that players can think of and have systems in place to defend against it. That is why it takes 6 seconds (or a few seconds less) to submit a bet live. That timeframe will be used to check if an event has happened that significantly changes the outcome of the match. In case of football those are goals, penalties, and red cards.
Even if there is a human error that has resulted in the markets to stay open that players take advantage of. I have seen bets be voided and bookies adding information such as
All bets made between 10:12:40 - 10:12:59 are voided due to a technical error.