In any case, while a hypothetical Witch Hunter reading Mathilde's diary could likely point to several violations of the "no touchy Dhar" rule, I do think that it's an important character beat for Mathilde that she has so far refused to break that rule by
her own standards.
Even if we did our Second Secret testing in a perfectly isolated area where nobody would ever track it back to us and our belt burnt away all the Dhar before it could have any magically corruptive effect, that would still be Mathilde deciding that Yes Actually I Can Handle The Dark Magic, It Needed To Be Done, Why Can't They See That.
And from there it's only a short hop to I'll Show Them All.