For convenience of readers, the text is:
The Migration or Importation of such Persons as any of the States now existing shall think proper to admit, shall not be prohibited by the Congress prior to the Year one thousand eight hundred and eight, but a Tax or duty may be imposed on such Importation, not exceeding ten dollars for each Person.
which apparently describes the international slave trade in a way that is perfectly opaque to me.
Did the s-word make slavers feel awkward or what?
More or less. The original versions of these clauses were usually more explicit, but they were revised around so that the Constitution would not grant explicit recognition of property in people. This distinction was mostly rhetorical at the time, and an issue about which some of the Virginians were squeamish, but it created space for an antislavery theory of the Constitution later on. Of course they didn't know that was coming at the time.
How prevalent was that squeamishness? It's hard to say because it's so trivial that proslavery enthusiasts didn't have a real reason to pitch a fight over it and so force firm lines to be drawn.
In actual practice, the Constitution is extraordinarily vigilant with regard to slave property and that's very much by design. Hence the later antebellum debate between Garrisonian types who took the Constitution (correctly) as a proslavery document and damned it as a covenant with death and people who believed there was more room for constructive engagement within the Constitutional framework. The obvious guy here is Lincoln, but the big innovators on the side where James G. Birney and Salmon Chase back in the Liberty Party days.
Sources
Free Soil, Free Labor, Free Men by Eric Foner
Slavery's Constitution by David Waldstreicher