What it did was ask me to "log in" (there was no prompt to sign up), which I did with our internal credentials. It also asked for an email address, which in retrospect was foolish of me to provide, but that was the only indicator that something was not right. It did not tell me where I would be publishing to.
When it was done it said the package was published. I checked the internal repository and it was not there. Instantly felt that horrible feeling in my stomach that everyone knows. I went to npmjs.com and found out that it had helpfully created an account with the internal credentials I provided, the credentials that were given in a prompt that said "npm login".
What saved me was that the package was inside an organisation ("@foo/bar") and those are not free on NPM, so the package was not published, even though Yarn said so. The user account was created, though, so I had to recycle that password and send email to NPM support to get the account deleted (there is no way to delete your account yourself, sadly).
The moral of this post is that when I saw this tweet I did not laugh at the company's foolishness in publishing internal code on NPM, because I almost did that myself and saw how easy the mistake is to make. (Of course, the rest of the tweet with the DMCA stuff is foolishness.)