I think it should be moved up.
Let's look at the total overall stats of Sigilyph, 490. Other Pokemon that don't evolve who are on the same base stat as Sigilyph are Kangaskhan, Tauros. Both are Intermediate.
As Winter said it's a bit like Absol, however Absol has lower overall stats, at 465. Other Pokemon who are on the same base stat as Absol are Sawk, Thor, Magneton, Bisharp, all are Intermediate.
What I am saying is that Sigilyph has same base stat and also has higher base stat than other Intermediate Pokemon. Just a thought to think about.
My two cents.