The 80% figure is clearly too high. Even if you only look at a few of the other major Arab countries such as Syria, Iraq, Saudi Arabia, not to mention the smaller population Arab countries and all the Arabs living in many other countries, the percentage in Egypt comes out to be much lower than 80%.
Wikipedia says that there are about 300 million Arabs of which 82 million live in Egypt, so that makes Egyptian Arabs not even 30% of the total.
Apparently it is not always 100% clear who is an Arab.
It is probably where 80% of the Coptic Christians live.