With all the respect, how can you be 100% sure that GRE Subject Test has never helped anyone in the admissions process? Have you verified this with all the admissions committees from schools of various levels of competitiveness and selectivity? Is it absolutely not possible that all other things equal, adcoms will be more assured about a candidate's math ability because they've done well on the Math test? For example, if two candidates with not many math courses are on the table with all other credentials similar.
Especially if an assistant prof who's reviewing this particular application is familiar with the subject test (which is not at all unlikely).
A claim that it usually doesn't help much and thus is probably not worth the effort required is one thing. Claiming that "even that score will not help" is another thing. You simply can't know that with 100% certainty.