Well you could take my word for it.
If you don’t find me credible - MY FEELINGS ARE HURT 
- then here’s a moderator saying the same.
You could also do a few google image searches and find different Proteus cards who have the special skill maxed but have different hero levels (and therefore different attack stats). If I am wrong, then they will show the same DoT.
Searches on this forum regarding Proteus and DoT will give you the same answer, as will searches for any other hero with DoT, like say Colen.