When no one else is able to duplicate their results, the reasonable conclusion to reach is that their sample was contaminated.
Except that the National Center for Science education was citing a 1981 study from Chadwick:
"When later, more comprehensive and careful studies failed to reproduce these results, it was concluded that Burdick's work was simply a case of contamination by modern pollens (Chadwick 1981)".
Read the link. I repeat; this is NOT the Burdick or Chadwick work. It is more recent. The pollen was found and the collection techniques are throughly documented.
Cordially,