Hi!
The Vivaldi browser website states that Vivaldi packages in Manjaro are managed (or something similar) directly by the Vivaldi team.
Is that correct?
Does that mean Vivaldi is officially included in my beloved Linux distribution, Manjaro?
Yes
sudo pacman -Syu vivaldi
But it is also available from Arch Linux but before that happened it was in the official Manjaro repo.
Welcome to the forum! 
Actually, the package we use comes straight from the Arch repository — and thus, it is Arch which packages it — but the software itself is Freeware, which means that it’s proprietary software, albeit that it’s freely downloadable. As such, the source code is closed-source, and is indeed only maintained by the vivaldi developers themselves. 
Yes, we do carry it in the repositories.
Furthermore, the Wikipedia page has the following to say…
In order to install it (if you don’t have it already), all you need to do is type the following command in a terminal window…
sudo pacman -S vivaldi
Hi! Thanks for the info.