Well if you want reasoning, since there is no written contracts on paper, typing the words and posting them should be enough of evidence.
How do the escrow providers operate if not by posting? Though they provide signed message as proof of holding the funds, not as proof who is posting, so the post alone is logically considered a written contract.

They have received their implicit benefit out of this sponsorship, the participants however did not receive the promised benefits.

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On a side note, love how you guys are worried about leaving a tag and want to have solid reasoning. Wish we had that in our universe where I'm from.😅