There are quests with scaling XP rewards, but to my knowledge, The Death of Chivalry isn't one of them.
The prayer lamp you mentioned awards a fixed 60,000 Prayer XP.
The stats of the enemies you fight during the quest do scale to your level, though. Perhaps you're thinking of that? IIRC defeating them doesn't award combat XP, so you don't have to worry about that scaling.
You find the additional level-locked lamps after completing the quest by looting the tomb where the quest takes place (you monster). You can come back and claim them if don't you meet the requirements, so no worries about losing out.
This achievement's page quite handily summarises the post-quest XP rewards:
https://runescape.wiki/w/Chivalry_is_Dead
More details about the additional post-quest rewards here:
https://runescape.wiki/w/The_Death_of_Chivalry#Rewards