The last stitch of the Mishnah in Demai 3:2 reads as follows:
היה עומד ולוקח וראה טען אחר יפה ממנו, מתר להחזיר, מפני שלא משך.
If one is ready to purchase something [but has not yet made an acquisition on them] and finds a better bundle, he may back out, since he has not acquired them. (Translation based on Bartenura.)
How is this different than the following Mishnah in Bava Metzi'a 4:2?
כיצד משך הימנו פירות ולא נתן לו מעות אינו יכול לחזור בו. נתן לו מעות ולא משך הימנו פירות יכול לחזור בו. אבל אמרו מי שפרע מאנשי דור המבול ומדור הפלגה הוא עתיד ליפרע ממי שאינו עומד בדיבורו.
How is this? If one acquired fruit but did not give him payment, he may not back out. If one paid but did not acquire the fruit, he may back out, but [the Rabbis] said, "He Who punished the people of the Generation of the Flood and the Generation of the Dispersion will ultimately punish those who don't stand by their word."
Further, while the Halacha in Demai is technically true, it's only half the story: it doesn't address Chazal's curse. To be fair, it is unclear whether the Mishnah is dealing with a case where the money was given or not, so it is possible that the Mi Shepara wouldn't apply there. However, the first question still stands: either it's dealing with the same case or it's dealing with one even more obvious and is still redundant.