Spell damage amplification does work for non-hero units (well, at least for the Spirit Bear). But their tooltips cannot show the bonus when holding ALT

First off, a proof that spell damage amp does work for non-heroes (or the bear at least). First root is withiout Kaya, second one is with Kaya.

And here is a tooltip comparison with and without Kaya. In both cases, it shows 60.00. You can also see how the HUD does not show the 10%. It still shows 0.0% for the bear with Kaya