The Gemara concludes that one cannot recite Zimun retroactively (after one has already recited birchas hamazon). Why would one think he may do so – since zimun is merely a form of Birchas hamazon, which he has already recited?
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Zimun is a separate part of birchas hamazon with a different source pasuk. If it’s thought of as a mideoraysa because of its pasuk, then perhaps it has a different chiyuv as well. Perhaps Merimar was saying that, in fact, it’s not a mideoraysa and therefore once you have lost the opportunity there is no requirement to go back and say it.