You’re right, America did bad thing, clearly this completely overrides the wrongs of other countries
The first commenter is talking a hypothetical scenario of socialism being bad, so the second commenter (the one you responded to) responded with actual example of that same hypothetical scenario happening, but except by a capitalist power (the US). I don’t think your response makes sense at all here.
No, his response is calling out the whataboutism fallacy. The US doing something bad does not in any way, shape, or form make socialism any less shitty. It’s poking fun at the delusional people who still think it’s a good ideology despite the overwhelming evidence.
Calling something “Whataboutism” infers a belief in American exceptionalism. You should question that belief.