The phrase "were tantamount to" means things that are equal or alike in effect, value, or meaning. Therefore, the antonyms for "were tantamount to" would suggest the opposite, or things that are radically different or opposite from each other. Antonyms for this phrase could include contradictory, incongruous, disparate, dissimilar, divergent, unequal, opposite, opposite in meaning, opposing, or unlike. Using antonyms in a communication creates opposition and contrast, making it easier to explain and understand things in more detail. When a combination of antonyms and "were tantamount to" are used together, it adds depth and complexity to the language, creating a more interesting and engaging experience.