You are confusing different aspects of the Dangerous offenders provisions. Violent disorder is a specified offence under the provisions, not a serious specified offence. A specified offence has to carry a max imprisonment of 2 years or more, where the serious specified offence has to carry max 10 years imprisonment or more. Violent disorder carries a max imprisonment of 5 years. An IPP (imprisonment for public protection) concerns serious specified offences, of which violent disorder is not one. Specified offences (like violent disorder) may attract an extended sentence, i.e, a period of custody and extended period on licence if the offender poses a significant risk of harm to the public. The circumstances of the offence and the previous convictions would be taken into consideration.