Majuub

Actually, the South Kaioshin might have been stronger than SSJ3 Goku or Chibi Buu. Chibi Buu chose to absorb him instead of just killing him, we know Buu absborbs only when he is outmatched, if he is stronger, he kills.

Shin Buu absorbs Gotenks to defeat Gohan

Shin Buu absorbs Gohan to be the strongest ever because the fusion worn off and Gohan was now strong enough to kill him

Shin Buu absorbs Vegeto because he was too strong

Now, Chibi Buu fought Goku but didn't even think of absorbing him, still they were dead even, yet he chose to absorb the south Kaioshin, it just means the Kaioshin was extremely strong.

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