The phrase "threw doubt on" can be interpreted as casting suspicion or uncertainty on something. Conversely, some antonyms that might be used to replace this phrase could include "confirmed," "affirmed," "validated," "supported," "endorsed," "verified," and "attested to." These words all suggest a sense of reinforcing or backing up the original idea or statement, rather than challenging or questioning it. Using antonyms in this way can help to create a stronger sense of clarity and conviction in your writing or speech, and can make your arguments more persuasive and authoritative.