He didn't do a visible power up though before he tanked Baby's RDB and went on to fight GO Baby. He could just have been holding back his effort.
Shouldn't Baby have known he had no chance then?
Episode 35
Time: around 9m55s
Context: Baby is seemingly beating Super Saiyan 4 Goku
Baby: (thinking) "All right! I can take him!"
Episode 35
Time: 13m35s
Context: After Baby's Revenge Death Ball does nothing to Goku
Baby: "I-impossible! I could tell it got you! I'm sure I must have hit you!"
Goku: "You ain't able to beat me."
Baby: "W-what?!"
Goku: "There's been too much difference placed between our levels of power."