No, if they are heterozygous, then they are, by definition, not purebred Scottish Folds. Purebred means an established pedigree of breed on both lineages. If one parent is just a short-hair, it is not a purebred Scottish Fold.

That is absolutely untrue. Both upright and folded eared Scottish Folds are considered purebred Scottish Folds. I have no idea where you heard otherwise but you are using the term purebred incorrectly here. Purebred refers to the BREED of the parents. Both upright and folded eared Scottish Folds are registered Scottish Folds and considered one in the same. Having upright ears does not suddenly turn the cat into a separate breed! Almost no one is breeding double homozygous folded cats anymore, so using your logic NO purebred Scottish Folds exist anymore. It's just absolutely incorrect.

Source: I am a cat fancier and have been exhibiting purebred cats in CFA shows since 2008.