Stacking Guinsoo's Rageblade is technically considered an "on-attack" effect, not an "on-hit" effect, so Sated Devourer won't affect it at all.
Now the difference between the two is that an on-hit effect applies to auto attacks after it actually lands and deals damage. If the attack is dodged, blocked, or it misses, than any on-hit effects will not apply.
An on-attack effect, on the other hand, will apply to auto attacks at the beginning of the attack, rather than after it lands, and will apply even if the triggering attack is dodged, blocked, or it misses. An example of this is the bolts from Runaan's Hurricane.
Stacking Guinsoo's Rageblade is coded similarly to the bolts from Runaan's Hurricane. Both are on-attack effects and both are unaffected by Sated Devourer's phantom hit. The full stack AoE damage, though, is an on-hit effect, so it should work with Sated Devourer.
This is also the same reason why abilities that proc on-hit effects, like Ezreal's Q, Irelia's Q, or Gangplank's Q, won't contribute to stacking Guinsoo's Rageblade. These abilities work with on-hit effects, but they don't work with on-attack effects.