No they are not the most Authentic & there author did't ever claim that. They narrate with isnad/chain with narration or refering to it collecting collection worthy narration. In islam as ordered by the quram & Hadith no narration could be regarded as authentic without proper scrutiny, its haram & more sinful in case related with the prophet.
As far authentucity concern there are very few seerah book that can be mentioned. One such book is from shykh Ibraheem Ali(rh) & other is the seerah book of Muhammad Salih al Munajjid.
Ar Raheequl Makhtum(translated as The sealed Nekter & Asah us Siyar of Abul Barakat Abdur Rauf Danapuri rh. are also mostly authentic & Reliable.