Wonderfl , stop casually copy and paste info from article A to B without checking or please drop your wild assumption. POE shuffle map boss in the past so that the patch note regarding Atoll Map may not relevant to Puruna. According to old (year 2013) youtube video, Champion of Frost was the map boss of the original map series' Atoll . Or instead you can check page history of Map boss to start with, despite past edit may still wrong.
I haven't dig all the old gameplay recording in youtube to check, but the old (POE 2.0) map boss of Underground Sea Map was The Idol Beyond which use lightning skill, while Puruna i am still digging info . After shuffle, The Underground Sea Map item is not replaced, still the same name, but the layout is replaced as well as boss. Underground Sea Map was using Underground River Map layout and then it was swapped between River and Sea. And the boss, for sure it was swapped with other map (Underground Sea Map now has Merveil as boss; still not sure The Awakening map series when it was release, the bosses of Atoll/Underground River are .)
- https://www.youtube.com/watch?v=HpLfCekNxuo Mountain Ledge Map (67) - Level 75 Duelist Oct 5, 2013