Flower size is debatable.
I had bought one in May with small flower and seller said it is Niveum.
The seller could be wrong. Not all sellers know what they are talking about. Also, there is no one, single characteristic that distinguishes thaianum from niveum. The identity is a result of consideration of the sum of the many parts, that makes thaianum stand out as separate from niveum.
It is similar to the thread poster.
I had the flower flattened and dried up in my book.
So? That's not evidence of anything. It can certainly be reasonably said that all thaianums are similar to all niveums. The difference between the two species is in the details, not the similarities.