It appears that referring to non Muslims who enter Islam as being “reverts” as opposed to “converts” is terminology unique to English speaking Muslims from the West.
The argument put forward by “revert” supporters is the hadith which states that all humans are born on Fitrah which is Islam and thereafter change their belief due to their respective upbringing. According to Shaykh Abd al-Fattah al-Yafi there are some Muslim scholars who hold this view.
As an aside – when did the phrase “Let me revert back to you” come in usage? Is it not supposed to be, “I will reply back to you?”
Coming back to the topic at hand, it is clear that numerous prominent Sunni scholars held the view that Fitrah (natural disposition) referred to in hadith is not Islam, and thus by extension for English speakers the term “convert” would be more accurate. One evidence of many is the following hadith recorded in Sahih Muslim 4/2048:
The Messenger of Allah (Allah bless him and give him peace) said: Every person is given birth to by their mother upon the Fitrah, thereafter their parents make them Jewish, Christians or Magians, and if the parents are Muslim then the person will be Muslim…
Imam al-Nawawi in his commentary said:
The strongest view is that the meaning is that every child is born with the natural disposition towards Islam.
Imam Ibn al-Athir said in Al-Nihayah (1/247):
The meaning of the hadith is that every child is born with a type of natural disposition which is the Fitrah of Allah Most High and that it inclines towards the acceptance of truth both naturally and in obedience.
Other scholars who supported this understanding are: Ibn Abd al-Barr, Ibn Atiyyah, al-Qurtubi, Ibn Qutaybah, Abu Shamah, al-Ayni and others, and Allah Taala knows best.
Source: Mawahib Karim al-Fattah of Shaykh Abd al-Fattah al-Yafi 1/531 onwards.