We're not talking about whether slavery existed, but rather about the question: why didn't our Founders use words like "slave" in their 1787 Constitution?
Why did they replace "slave" with euphemisms or indirect language?
The obvious answer is, our Founders well understood that slavery was both wrong and disgraceful, and so could not be called by its real name in their most polite of documents, the 1787 US Constitution.
As for Charles Pinckney, he was directly involved in the Constitution's Fugitive Slave Clause, and so obviously participated in obscuring direct references to slaves, which are there called a "Person held to service or labor".
So, it looks to me like even SC's Charles Pinckney understood the 1787 American squeamishness about words like "slave", "slavery" or "enslaved".
A squeamishness which was totally abandoned in the South by 1860.
Not just the South. Look at my earlier post about how President Lincoln blabbed about slavery being in the Constitution to his national audience on inauguration day.
But your point about northern hypocrisy stands. At the time Lincoln was speaking frankly about corralling and returning runaway sla*es he was also dangling the northern Corwin amendment carrot which used the euphemisms you find objectionable; all to protect tender Yankee sensibilities.
You have a right to be rankled.