Thanks for raising this Goffi,
I can see how "take responsibility" can be interpreted as being too vague. If it is interpreted as imposing legal liability on the individual author, that could be problematic. I don't think it obviously does, but prevention is probably pretty easy and cheap. I suggest to slightly modify your suggested text, into this:
"You must understand and verify every part of your contribution, and ensure that material derived from other sources is properly attributed."
I'm less keen on "ensure that your work is original", since XEPs necessarily build on existing specifications and prior work, and "original" could be ambiguous.
This wording keeps the requirement tool-neutral: it applies equally to copied text, AI-generated material, machine translation, etc. The important point is that the author understands and verifies what they submit, which was the motivation for the PR in the first place.
Kind regards,
Guus