Is that based on an eisegesis of Revelation 12:17?
No it is based on the history of God's people as recorded in the OT. Due to stiff necks and hard hearts it has only been a small few in any given time throughout history out of those who profess.
And there is Jesus' own words also.
Matt 20:16 So the last shall be first, and the first last: for many be called, but few chosen. KJV
Matt 22:14 For many are called, but few are chosen. KJV
Matt 7:14 Because strait is the gate, and narrow is the way, which leadeth unto life, and few there be that find it. KJV
Luke 13:23 Then said one unto him, Lord, are there few that be saved? And he said unto them, KJV
Luke 13:24 Strive to enter in at the strait gate: for many, I say unto you, will seek to enter in, and shall not be able. KJV
Rev 12:17 is an interesting text though. What do you see? Remnant means remaining ones by the way.
Rev 12:17 And the dragon was wroth with the woman, and went to make war with the remnant of her seed, which keep the commandments of God, and have the testimony of Jesus Christ. KJV
Rev 14:12 Here is the patience of the saints: here are they that keep the commandments of God, and the faith of Jesus. KJV
Rev 19:10 And I fell at his feet to worship him. And he said unto me, See thou do it not: I am thy fellowservant, and of thy brethren that have the testimony of Jesus: worship God: for the testimony of Jesus is the spirit of prophecy. KJV