Your overly-clever lawyering here cannot erase the fact that nobody at the time understood it that way.
Indeed, if anybody in the world could successfully challenge Pennsylvania's right to abolish slavery in, say 1790, it would be President Washington, who knew exactly what the Constitution's words were intended to mean.
And Washington did not attempt to impose your too-clever interpretation of the Constitution's plain words.
Instead he obeyed Pennsylvania's abolition laws.
God forbid that anyone should be lawyerly regarding the law.
Whatever way they "understood it" is irrelevant. Courts have ruled over and over again that "intent" is less relevant than the text of a law. Who can say what was the intent of the ratifying conventions in Virginia or South Carolina when they approved the law?
Perhaps they understood it to mean exactly what it says; That no state law can interfere with returning a laborer to the person to whom the labor is due in accordance with the laws of his home state.
Perhaps you are the one trying to do the clever lawyering by asserting a meaning that cannot be discerned from the text itself? You put conditions on the law that are not apparent from the text of the law.