Translation:Antoine is absolutely bad in gymnastics.
the pronunciation is correct. The reason it sounds like nulle is because the next word starts with a vowel so the l sound is carried to bridge the gap. Similar to how "vous mangez" sounds like "vou mangez" but "vous avez" sounds like "vouzz avez". The pronunciation of the word is affected by the word after it
The english translation is 'absolument nul'. It just doesn't sound right to say 'absolutely bad in' as a native english speaker I would never say it. Myself I would probably say 'Antoine is absolutely useless at gymnastics' or 'absolutely rubbish at'. Absolument also translates as completely so 'completely hopeless' 'completely useless' should be acceptable too. Agree with everyone that it should be 'at' and not 'in'.