What's so special about Justin Bieber that disqualifies him from counting as someone who became famous on YouTube?
Why limit yourself to the top 50? 0% of all Twitter users are primarily known for their tweets. Tweets are, by design, unsuitable for publishing content. They're useful for advertising content you publish somewhere else.
That's just an example of using Twitter to advertise your other content.
I agree with you that once you have those followers, you can leverage that with other content. But building up your audience mainly needs to happen through other channels.
This is false.
The original comment was a bit miseleading, but you have to admit that there is no equivalent to Pewdiepie on Twitter.
They would still be popular though.
YouTube hasn't even been that popular in South Korea until recently.