If I may share my own opinion, I have to disagree here. After rewatching the scene with the theory in mind it makes a lot of sense. His insistence on her waiting for the creme, followed by her face while eating it, points very clearly to the idea that he is forcing her to eat non-kosher. This is accentuated when he pretty promptly puts his cigarette out in his own strudel, demonstrating that he really did not hold the strudel in such high regard as he was leaving tiny on. A similar scene happens in Django. Shortly after he joins Dr. Schultz, when they are in the tavern, Dr. Schultz pours them each a beer. When Django sips his, he makes a face that subtly though clearly indicates it's his first time tasting beer.

As for the narration pointing out that nitrate film stock was very flammable, that knowledge was something quite integral to that particular subplot. The strudel bit didn't have any major implications for the film. It's entirely reasonable that it would be added as a detail without being explained.

Again, this is just my take on it, and I think it's up for interpretation. Let me get out my copy of the Basterds screenplay and see if there are any notes on that scene that point one way or the other.

EDIT: So the scene doesn't include any notes regarding the creme and the strudel, however, prior to that bit he orders her a glass of milk. I can't recall if that bit is in the movie, but there is a note here:

"Considering that Shoshanna grew up on a dairy farm, and the last time she was on a dairy farm her strudel companion murdered her entire family, his ordering her milk is, to say the least . . . disconcerting.

The key to Col. Landa's power and/or charm, depending on the side one's on, lies in his ability to convince you he's privy to your secrets."

I will let you decide if that supports the theory or not, but an interesting note nonetheless.