I'm guessing it's because Hippie Zilean move speed 9999, actually interacts with GA coding to make his ult apply, which his Skill + scaling replaces the default value (the % one) so healing done is considered done by the item rather than Zilean.
If they made his heal show up as healing done, then it is easily done by making GA count as healing done. (which they don't do, because it unfairly makes your score better for dying and triggering GA)
If we were to apply it as an actual heal, then they would need to write more code to separate Zileans heal for normal heals. Which unnecessarily makes the coding more messy. So fortunately, between Normal heals (which can be weakened) and GA (uncountable heal) Zilean got the GA one.
We will see the return of the infamous 50% reduced heal when the ADC patch comes through, so placing Zileans ult into the "Normal" heal category would actually be a nerf.