Has anyone from the Biden camp stated that? Whether it's factual or not is irrelevant to your question if no one from the Biden camp is saying he is in "decline".Nobody has asked her when she knew Biden was in decline, and why did she choose to not to be forthcoming about it?
"Is president Biden in cognitive decline?" Would be the appropriate question. One of which I believe she has been asked and has answered. You just don't agree with that answer.