The arrangements outlined in article 11 would not necessarily be in the risk assessment. The risk assessment would show what general fire precautions are required (and what precautions are already in place) to protect relevant persons, (Then article 9 is complied with) the RP should put those measures in place, (Various articles will be complied with by this act)
Article 11 then asks for the 'arrangements' covering the ongoing aspect of maintaining and monitoring the suitability those measures, with the need to record these arrangements following the same criteria as article 9.
I think this is better thought of as an explanation of 'how' a RP is going to comply. i.e. RA identifies arson as a risk, and suggests a method of dealing with arson. By creating a written arson policy (i.e. This is how we are going to deal with the risk... This is how often we will review the suitability...) this is complying (in part) with article 11. By actually instigating the policy in reality, the RP is then complying with article 8. (So far as the arson risk is involved)