When Sky Sports are literally the only outlet reporting a buy out clause, I would err on the side of caution.
No decent journalists/publications have actually confirmed the clause.
Until someone like John Percy confirms it from Villa's side or, if we progress with a deal and someone like Lee/Mullock/Gaughan/Blackburn confirms it from City's side, it's all conjecture.
I highly doubt you're getting Villa briefing their journalists that Grealish isn't for sale at any price when he's available with an undervalued release clause.