Thanks Joseph
The source I was looking whe I asked earlier, was directed to cheerybim who said “This is based on the fact that for the past fifty years, the intermarriage rate, outside of Orthodoxy, is close to 75%.”
You are right about the the interrmarige rate among non-orthodox being higher than the 13% I came across for the reason you mention (However not much higher since in 2013 PEW estimates 10% ORthodox, which was less in 1970. So the vast majority of jews were not frum and the intermarrge rates I came across ranged 13-17% so even if we “correct” for the orthodox included in that denominator it isnt a large number that needs to be excluded thus the numerator wont rise by much). But there is no way it could have been 75%. Even in 2013 the numbers arent quite 75% Though they are close. And sadly 75% is certainly believable in 2013