It's quite obvious that it means "running in the browser engine" rather than "has the technology implemented in a native language". But sure, you're technically correct.
Not sure about the Opera Bittorrent support, but WebTorrent is years old, don't be fooled by the fact it's been posted here today. I remember first trying it at least 5 years ago.
Not quite. Opera had built-in support for Bittorrent for quite some time: https://www.reddit.com/r/todayilearned/comments/ec79j/til_yo...