If they'd made a new tool/project called "yarn2", leaving "yarn" alone, that would've been fine.
You install it into your repository, and it gets vendored.
The yarn 1 binary on your machine will use yarn 2/3 if it’s in the repo, otherwise it will just run yarn 1 as normal.
If they'd made a new tool/project called "yarn2", leaving "yarn" alone, that would've been fine.