That Fallon segment essentially like a person in a school being hit by a small rumor then multiple people in his class keep asking his confirmation with whisper. Then he's so fed up, go to school broadcasting room and announces his rumor and the answer with speakers. Now the whole school know.
Still don't buy it (I dunno why tho) that Hammer is already cast.
But I must admit that if I really thinking about it, he has way too many advantages.
- Look wise, it's obvious that he is CBM material.
- He's a familiar face but not overexposured.
- Has experiences, including indie and action movies.
- Pretty acclaimed and had been working with popular directors.
- Was Batman, WB love to come back to past considered actors like with Henry and Amy.
- Matt Reeves, Geoff Johns followed him.
- He knows and is a friend with some DC actors.
- Peter Safran likes him and championed him to his project before.
- The Rock and his producer friend also like him.
- Rumor that WB want a specific actor and some scoopers said that apparently was him.
- His casting rumor actually not received negatively by many.
- Actually interested in the role and seems committed, somehow without sounding desperate.
etc.
He's almost like an OP game character.

I don't know if it will be him, but I do think he was considered even if things fall apart.
That being said, nah, don't think the role already his. But what do I know.