The answer is twofold:

First: the ḥadīth is not authentic; indeed, it is baseless. The eminent scholar al-Subkī said, “I have found no chain for it—neither sound, weak, nor even fabricated.” I add that it has only been reported with the wording, “… ikhtilāf aṣḥābī lakum raḥma (the differences among my Companions are a mercy for you),” and, “aṣḥābī ka-n-nujūm, fa-bi-ayyihim iqtadaytum ihtadaytum (My Companions are like the stars; whichever of them you follow you will be rightly guided).” Neither version is sound: the first is extremely weak, and the second is fabricated. I have treated the matter in detail in Silsilat al-Aḥādīth al-Ḍaʿīfa wa-l-Maudūʿa (nos. 58, 59, and 61).

Second: in addition to its weakness, the report contradicts the Noble Qur’an, for the verses that forbid religious discord and command agreement are too famous to require mention. Nonetheless, we shall cite a few examples. Allah the Exalted says, “And do not quarrel among yourselves, lest you lose heart and your power depart” (Al-Anfāl 8:46). He also says, “Do not be of the polytheists—of those who split up their religion and became sects, each party rejoicing in what it possessed” (Ar-Rūm 30:31–32). And He says, “They will not cease to differ, except those upon whom your Lord has mercy” (Hūd 11:118–119). If those whom your Lord has shown mercy do not differ, while only the people of falsehood do, how could disagreement ever be a mercy?

It is therefore established that this ḥadīth is unsound—both in chain and in content—(1) and it is plain that it may not be used as a pretext for refraining from acting upon the Book and the Sunnah, as the leading Imams have commanded.