I’m having a lot of trouble finding a source that backs up this position. Everything I’m reading says that loopholes are typically oversights, not intentional inclusions.
That being said, the 13th amendment’s allowance for prisoner slavery is not a loophole at all, it’s an explicit allowance. Loopholes are not explicit, that’s kinda the whole point of them. It’s a bit like saying that the standard deduction on your taxes is a loophole. It’s just an explicitly defined feature.