I think it depends how he means it.
You're right that the gospel is intelligible and comprehensible - otherwise no rational adult could be saved (since you can't be saved without understanding the gospel).
But if he meant in the sense of, "no one can understand the depths of God's love in the gospel," or something along those lines, I think that's true.
In one sense we understand it (and Christianity is a reasonable faith), but in one sense there are things about God and the gospel we'll never fully understand.