"No bull" in this case being short for "no bullshit" - no misleading, exaggeration, or lies. It's an idiom, possibly an American one but I'm not sure. They're u

"No bull" in this case being short for "no bullshit" - no misleading, exaggeration, or lies. It's an idiom, possibly an American one but I'm not sure. They're using it here as a pun.