You are right – when looking up the auxiliary_meanings of 巨大 via the internal API, there is an entry for “giant”:
(await fetch("https://www.wanikani.com/extra_study/items?ids=5013").then(r => r.json()))[0].auxiliary_meanings
// results in:
[{"type": "warn", "meaning": "Giant", "message": "That's a bit ambiguous. Can you give us another meaning?"}]
If you answer with “giant”, you get a shake and the specified warning message (unless you have “giant” added as a user synonym – in that case WK immediately marks your answer wrong because of a bug).