The phrase "are tantamount to" is most commonly used to suggest that two things are equivalent or have a similar meaning. Therefore, it's antonym should express the opposite, meaning that they are not equal or do not have similar meaning. Some possible antonyms for "are tantamount to" might be "are disparate from," "are not alike," "are not equivalent to," or "are not similar to." Using these antonyms in place of "are tantamount to" would convey the idea that the two things being compared are very different or not related.