They are. Try it out. Not in late game when they swarm your base on an open map but before that point Goths have little answers to Incas.
Plums are the better general unit, no question about that. But against infantry slingers are better in most situations (less investment required, faster to get to, more attack output).
But they don’t really need it. If you want a buff for incas it would be better to ask to change their team bonus, or to put more food in their starting lama.
Maybe he was referring to the starting lama, which is an useful bonus, a bit like tartars and lithuanians, but you can also use it to explore in dark age.
Maybe high elo players and pros are able to use it, but for us common mortals, yes, it’s pretty useless…
I respect that but buffs aren’t the cream on your favorite cake they are means to improve the performance of weaker civs. If you like them just play them. They are a decent civ and (at least for a meso civ) a pretty well-rounded civ.