I asked for absolute proof. This is not absolute proof. Hell, the title of paper even openly implies its uncertainty, “All Signs Point to Russia…” (emphasis mine).
But I can offer an explanation, though the burden of proof does not rely on me, and I will even quote the article you used as “proof.”
For example, Cyrillic could have been used as cointelpro. Your article even alluded to the possibility that, “The evidence linking the Guccifer 2.0 account to the same Russian operators is not as solid, yet a deception operation — a GRU false flag, in technical jargon — is still highly likely.”