It could be used to supress sibilants. However, it's not designed to be a replacement for a full de-esser. I tried it with an experimental build just now, with the minimum attack/release set between 1ms to 5ms, the effect is quite subtle. But you can easily tame general sibilance (e.g. due to mic saturation) with Grader.
The de-essers are among the most difficult tools to tweak accurately to reach a good result without loosing a bit on the general quality. And I think like you that Provoc Grader is certainly more generalist than a true de-esser then of course not as efficient as a true de-esser (and it's not either the same price, ha ha!) but yes it is probably a nice solution to tame general sibilances between a poor mic and an input stage... and in fact I was on another idea which was to enhance perhaps the quality of old vintage records without loosing the "ancient" taste of the voices. On the oldest videos and LPs of live performances of "cabaret" artists like Billie Holiday, Sarah Vaughan, Ella Fitzgerald, Sarah Vaughan, for instance, where there are always many sibilants due to the technical conditions of these old years, Provoc Grader could probably "refresh" the general quality of the voices in these very old records (and incidentally remove also a good number of cracks), resulting in a much better audio comfort... and perhaps to re-record them to use them as original "vintage samples", sort of winks in new tracks a bit jazzy.
Yes, you are correct, and that use-case would be nice to try. Grader is not a de-esser. Another use for Grader this is to treat harsh harmonics produced by the singer. Those times when the overall performance is good but gets too nasal or shrill at times. Sometimes a bass guitar would also do this, when a certain note would jump out and rumble. =)