"If not" - unorthodox usage?
Solution 1:
Both examples are correct and orthodox uses of the expression. In both cases,"X if not Y" means "maybe not Y but at least X".
In the Arcanus example, The writer presumably knows whether Arcanus is strong in body but declines to say that he is. This carries the implication that he is probably not strong in body. Nonetheless, in both examples "X if not Y" leaves "Y" unknown.