The judge mentions 'one blow manslaughter' cases, and they are quite common, and the sentence here seems in line with similar cases.
For instance, take this case:
www.getsurrey.co.uk/news/s/2050898_labourer_jailed_for_punch_death_manslaughter
"A LABOURER who killed a disabled man with a single punch after binge-drinking at the Black Cherry Fair in Chertsey was jailed for two-and-a-half years on Friday.
Joseph Sewell, 39, downed at least 15 pints of lager before launching an unprovoked attack on 59-year-old Keith Hanna.
Mr Hanna, who had speech problems and used a frame to walk, had been sitting in his car outside a fish and chip shop in Chertsey
CCTV footage played at the Old Bailey showed Sewell, along with Nathan Scott and James Hussey, walking towards Mr Hanna's car in Guildford Street at around 2.30am, and then beer being thrown into the vehicle.
Prosecutor Jennifer Knight said: "Towards the end Joseph Sewell can clearly be seen throwing a punch or jab at what must be Keith Hanna's head."
Mr Hanna was discovered by the landlord of the Kings Head pub, Thomas Spurling, and taken to hospital."
So in that case it was (a) a hate crime (b) unprovoked, and (c) the victim was left to die.
So in comparison a year less doesn't seem outrageous.
Obviously I would suggest much longer sentences for both parties, but I'm not sure this is a case of being lenient to DV perpetrators as much as a generalised tolerance of crime in this country.
Apparently the Lord Chief Justice wants longer sentences for 'one punch' deaths, in line with 5 years that you'd get for death by dangerous driving and that seems fair enough. When you choose to hit people there's a risk that you will kill them (the 'eggshell skull' in law) even if you didn't intend that, and you as the aggressor need to take responsibility for that.