I'm pretty sure the constitution was not written for slaves and no "originalist" stance would consider them. This is a silly "gotcha" that you just made up so you can get mad.
The 3/5th clause has been used by prior SCOTUSs to justify decisions, so it is not a gotcha. It's history...
On that note, Alito and Thomas had to use a pre-U.S. colonial law as their grounds to overturn Roe, so there is no limit to how far they will go to use "orignalism" to further their ideology.
That's the point.