"Standing firm" in the absence of solid evidence is unwise.
Is cross-contamination conceivable? Yes it is. One would have to be foolish to rule it out completely. On the other hand, one would also have no reasoned basis to take the opposite position and claim that cross-contamination was proven.
So one may ask:
"Is there any concrete evidence for cross-contamination in the Nguyen case?" No, there is none.
"Is there any concrete evidence for a source of anthrax other than cross-contamination in the Nguyen case?" No, there is no evidence of that either.
The known evidence leads to no definitive position on the source of her anthrax infection.
The original consensus of the experts was that cross-contamination was unlikely. The most common view among investigators originally was that she crossed paths with the perps in some way.
Only when no other source of anthrax was found did cross-contamination become the fallback position. There is no hard evidence in favor of cross-contamination as the source.