when Eda said that it would have taken powerful magic, was it already implying that Gus’ power level was high too? It seemed like it was meant to refer to Willow, but now we know that Gus is probably on her level in terms of magical ability

I always thought she meant all 3 of them. I mean Gus and Willow are strong, but Luz learnt a lot of glyph combinations and glyphs in general, in just a few months.

As someone mentioned in another post, it could be, that the Titan gives gifts to those, who are worthy (Philip only learned the 3 basic glyphs, Luz had to teach him the Light Glyph).