Hi Matthias,
Two-Headed Giant uses the default (London) mulligan rule, with the modification that your first mulligan is “free” and does not result in having to put any cards onto the bottom of your library.
I would note that the page you quoted largely skips over the details of the mulligan process – it simply says that mulligans beyond the first “result” in fewer cards, which is an accurate phrasing under both the Paris and the London rules.
If you'd like to flag the issue for Wizards webmasters to look at, I would suggest using this form:
https://magic-support.wizards.com/requests/newCheers,
Paul