Fallout – Is Sunset Sarsaparilla food

It only adds +9 radiation immediately when you eat it, then it gives you 5 HPs over 5 secods, 1HP every second.

You have to note that the (5s) description is only part of the HP +1 effect, not the Rad effect.

I'm not sure if consuming corpses directly counts. Even if it doesn't, once you've consumed 25, you'll gain the Dine and Dash challenge perk, which allows you to harvest Human Remains, which can be used from your inventory, and are most definitely food.

Ghastly Scavenger is unaffected by Dine and Dash, so if eating corpses directly doesn't count, then I suspect the same would apply for super mutants et al. The easiest way to test this by the way, is to take the perk, and check the challenge counter viewable on your Pip-Boy before eating a corpse.