She backdated the sentence to Jan 15, 2020, when he was remanded.
He added that the man had been exempted from national service because of this.
Four men have been arrested but the main attacker is missing.
He is even seen with his hands and legs tied as he is thrashed.
Mr Lim said the man had surrendered himself at a neighbourhood police centre in January last year, before he turned 21, and accepted responsibility for his actions.
The mobile phone video surfaced on Thursday, three days after the assault, and has been circulated on social media.