Catholic Answers on Luther's View of the Immaculate Conception

Catholic Answers posted an article defending their belief that Mary was entirely without sin, particularly when detractors question the impregnation of a teenage girl. While Martin Luther's view of the immaculate conception was only a passing comment, it represents a change in typical Roman Catholic cyber-treatments of the Reformer's Mariology. Apologist Trent Horn writes,  

Some Protestants might say that at best, this proves only that Mary was free from sin at the Annunciation, not necessarily since her conception. Martin Luther, for example, moved away from belief in the Immaculate Conception, but even in 1540, he said with regard to the Annunciation, “The flesh and blood of Mary were entirely purged, so that nothing of sin remained.” In response, I would just say that it seems arbitrary to say God chose this moment to give Mary grace rather than at any other moment and that the angel’s greeting, “Hail, full of grace,” signifies that her being full of grace was a part of her identity even before the announcement about the Incarnation.

Let's take a look at the citation used and conclusion reached by Mr. Horn of Catholic Answers.

Documentation
Other than the date 1540, no meaningful documentation is provided. The quote is from an English rendering of Luther's Disputation On the Divinity and Humanity of Christ (February 27, 1540). The Latin text can be found in WA 39.2:107.

This writing has been available online for many years via Project Wittenberg. This disputation has been included in LW 73, with the quote found at LW 73:267-268 (utilized below). The English translation in LW 73 was done by the same person who did the web version on Project Wittenberg.  

Context

Argument 10

Every man is corrupted by original sin and has concupiscence. Christ had neither concupiscence nor original sin. Therefore he is not a man.

Response: I make a distinction with regard to the major premise. Every man is corrupted by original sin, with the exception of Christ. Every man who is not a divine Person, as is Christ, has concupiscence, but the man Christ has none, because he is a divine Person, and in [his] conception the flesh and blood of Mary were entirely purged, so that nothing of sin remained. Therefore Isaiah says rightly, "There was no guile found in his mouth" [Isa. 53:9]; otherwise, every seed except for Mary's was corrupted.

Conclusion

Mr. Horn rightly says that Luther "moved away from belief in the Immaculate Conception." I would qualify this though by saying: Luther didn't just "move away" from it, he ceased believing it. It appears earlier in his life he accepted it, later in his life he did not.

If the sands of cyber time were reversed, this same quote, and others, were used by some of Rome's defenders to prove Luther held a lifelong belief in the immaculate conception of Mary!  It would be interesting to know what sources Mr. Horn used on Luther's view of the immaculate conception. Back in the early 2000's, it was common to find Roman Catholic webpages using Luther's statements about Mary as an apologetic tool against Protestants. I do not find the same amount of these webpages today. It seems to me the newer generation of Rome's defenders have learned from the errors of the older generation... of perhaps... they are better at using Google!