Set aside for the moment that the Founding Fathers absolutely wanted there to be a separation between church and state because they had just come from a land (England) where the ruler (the King) was the leader of the church (Church of England) and where they saw the abuses this caused.
Would Johnson and all the other "no separation" folks really be fine with the government meddling with their faith? After all, if there's no separation between church and state then not only can the church influence the state, but the state can influence the church. Get rid of the separation and the federal government could decide which holidays you observe and in which ways. It could proclaim what the contents of the prayer books are and when/how you pray.
Would they be fine with all this?
Of course, they assume that they will be writing the rules, but would they accept it if someone else was? Perhaps I, a Jew, would declare that they can't eat pig products. (In reality, I'd never impose my religious beliefs on others, but let's say I did hypothetically speaking.) Perhaps a Muslim Government Religious Committee Member would add a few rules. As would a Buddhist. Heck, let's get atheists and satanists involved as well. I'm sure they would love to write some "religious rules" that the Christians nationalists would need to follow. Would Johnson and company happily go along with this because "no separation between church and state?"