Here we would like to present to you an benefiticial answer from Shaykh al-Muhadith Shuaib al-Arnawut, which was recorded by one of his students from Turkey, Anbiya Yildirim.
It is taken from book of Anbiya Yildirim “Hadisler ve zihinlerdeki sorular” (p 269), printed by Ragbet in Turkey.
Shaykh Shuaib (hafizahullah) said:
As for takfir from modern shias toward some (people from) Ashara al-Mubashara, (and they saying that) “Abu Bakr, Umar, Uthman, Sad were disbelievers. Only Ali, Fatima and their two sons saved from disbelief”, when modern shias didn’t exist, this hadith (about 10 which were gladden by heaven) was in our books. When was written “Musnad” and “Saheeh al-Bukhari” shias were not like shias of our time. In that time when (someone said that such and such person) was shia, that mean he loved Ali more than Abu Bakr. There is nothing in love; man can love someone more than other. (In the past) there was no scientist (meaning) of tashayu like in this time. There was no ghulat (extreme view between shias) in the time of the mutakadimin. They (ghulat) emerged after the 4-th year of hijra. I can show you bios of shia narrators, there are shias of fist generation in Bukhari, Muslim, Musnad. So, Ahmad ibn Hanbal died in 241 h, and this hadith (about Ashara al-Mubashara) was recorded long time before his death.
So, before shias distinguished from ahlesunnah, (before) they made themselves as independent school, (before) they took idea that they believe in, as a (major) point of their movement, these ahadeth (about companions which were gladden by paradise) took their place in books. Did in that time were shias like these, so (someone could claim) that this hadith (about Ashara al-Mubashara) was fabricated (in attempt) to protect (companions) from their charges? So issue of takfir of muteahirin shias didn’t present in the time of mutakadimin.