Talk:Helios/Prime/@comment-17703786-20170223014200

Lore-wise, Sentinels are actually sapient bio-mechanical creatures discovered by Tenno according to the description of them that be can be found on completing the relevant Fragments entry.

So it can be reasoned that while the standard Helios is Corpus-styled, it's not actually a Corpus-made Sentinel. Thus the Helios is a pure Tenno creation, and thus can be primed.