The argument is that you should get deductions for abolishing slavery? Not really it’s a nonsense argument. And the reply that you don’t get to take credit is a straw man.

Imagine being severely injured in an auto accident due to a production issue where faulty parts were used. You sue the manufacturer for damages and they say well we fixed the problem on new models - do I get to deduct what I owe you? The answer is of course not- you’ve probably suffered a loss of income (and may even have inhibited your ability to work going forward), incurred significant expense in the form of medical bills, lost time managing the process, and endured a lot of pain; they are liable for the damages caused. But by fixing it they do prevent the circumstances that led to the lawsuit in the first place, so in that sense they do avoid paying out more for other cases that might have occurred. That’s the deduction.

Understand I’m not making a case for reparations - but if the argument is that it’s compensation for the damages caused by slavery. The answer to the question isn’t we don’t get to take credit for what our ancestors did, it’s that our ancestors reduced the amount of potential damages by abolishing it when they did. Again that’s not an argument for reparations itself- just against this dumb cartoon’s point.