Yeah I see what you mean now. From a Catholic standpoint when we say Jesus has fulfilled the law we don’t say that the “true meaning” has been brought out. By this understanding it would make it seem like the OT meaning was false.
The Catechism of the Catholic Church states, “The Law has not been abolished, but rather man is invited to rediscover it in the person of his Master who is its perfect fulfillment” (CCC 2053).
The solemnity of our Lord’s opening pronouncements and his clear intention of inaugurating a new religious movement make it necessary for him to explain his position with regard to the [Old Testament law]. He has not come to abrogate but to bring it to perfection, i.e. to reveal the full intention of the divine legislator. The sense of this “fulfilling” . . . is the total expression of God’s will in the old order . . . Far from dying . . . the old moral order is to rise to a new life, infused with a new spirit. (861)
So taking this into account we need to read St. Paul’s words through Jesus eyes not the OT.
Jesus tells us to Love God and Love neighbor we need to keep this in mind when we read St. Paul telling us not to be mismated with unbelievers.
We need to put these words to the test. If the “true meaning” here is to not mix with unbelievers is it possible to do this and still Love God and Neighbor? Well God wants us to share the gospel with everyone, including non-believers, is this possible if we avoid them? If we don’t bring them the gospel (obviously if we were given the gift of evangilization) are we loving the neighbor?
If we avoid the neighbor we aren’t loving them so this can’t be what St. Paul strictly means here. However, if we put ourselves into a situation that will pull us away from God then we aren’t loving God, so there has to be the happy medium which is what I said up there.
Hope this helps,