A Law given to Moses at Sinai refers to a halakhic law for which there is no biblical reference or source, but rather was passed down orally as teaching originating from Moses at Sinai. Such teachings have not been derived from any Talmudical hermeneutics, but known solely from the Jewish tradition.
While their very existence sounds weird [to me], that the Almighty found some very basic Halochos unworthy of being even hinted in the Torah, that should at least cover the basic Halochos, but at least the fact that those Halochos exist, that G-d passed some undocumented Halochos to Moses must be explicit in the Torah, IMHO.
I would compare it to what the Sages alluded in "לא בשמים היא" for their own authority to develop the Oral Tradition - we might not trust the Sages but at least it looks like the Torah "explicitly" allowed to do so.
Where does the Torah hint that there are Halochos not referenced by it in any reasonable way?