Actually it is because we are dealing with an factual data point (I), rather than someone using hyperbole by saying something is ONLY used for x. one data point proves that wrong
One data point disproves that it can't be used for purposes other than tax evasion and money laundering, it doesn't disprove that is commonly used for this purpose and little else.
so you'd accept one anecdotal comment about a transaction without any details as a fact, versus the well known numerous transactions bitcoin has gained infamy from in the darknet markets- which supposedly is evidential enough for the FBI?