(In general, perhaps you could just ask Rabbi Slifkin what he meant, as opposed to posing the question here.)
Just after you wrote this question (I think), Anonymous commented the following on the blogpost:
I think you mean ultraviolet light, not ultrasound. Ultrasound is what is used to look inside the womb.
Judging by the context (evaluation of stains), I would imagine this to be correct, as I think that UV can identify what is blood or what might be something else. We can only know whether or not this is the correct answer when Rabbi Slifkin responds at some point, which I will try to check later and update.
However, I believe that ultrasound could probably be used to some extent to determine Niddah status in some cases. For example, for unidentified bleeding, if an ultrasound showed certain information about the uterine/endometrial lining, it could be perhaps used in a Pesak (to determine that it has not come from a particular location). Also, there are some Hilchos Niddah that are affected by being pregnant for a certain amount of time, which, if an ultrasound could confirm that earlier, perhaps would affect those Halachos, but these have already been discussed by Poskim.
(A good website that discusses what a pelvic/gynecologic ultrasound shows is here, if this is of use to anyone in answering the question.)
Edit: Just Curious has responded to Anonymous as follows (there have been no comments from Rabbi Slifkin):
I'm pretty sure R' Slifkin means ultrasound. Transvaginal ultrasonography can be used to evaluate the lining of the uterus and thereby ascertain whether the source of vaginal bleeding is uterine or otherwise.