Here is the twitter thread from The Secret Barrister that clears up quite a lot of this.
https://twitter.com/BarristerSecret/status/1208096099287945216
In summary, the assailant in question seems not to have been involved in the actual assault, but to have merely indulged in verbal abuse (which on his part was not homophobic) and theft. He was a juvenile, subject to the very different juvenile sentencing rules, and it was a first offence, and I think he pleaded guilty. All this is taken into account.
As TSB says, it would be helpful if the courts would publish the sentencing remarks so that the press and public could understand the issues, rather than allowing speculation and public outrage to run unchecked.