How I know Mathilde didn't know Anastasia very well: One, there is no story of them ever meeting in any book. Also, considering that she was her father's former MISTRESS it's not likely she was a frequent visitor to the palace! Even if she did come, at some party, consider that the family was very private so the children were not commonly seen by all. And you had to be 16-17 to attend grown up parties. By the time Anastasia was that age, sadly, she was in captivity and no one saw her.
Also, if Mathilde was 95, that would have made the real Anastasia 65, and not looking much like she did at 15, the last time anyone outside captivity saw her! Much too old to judge by then.