His reply was not whataboutism; it was a legitimate and direct retort to your post that exposes that what you asserted was false.

You can literally replace the first four words in their sentence with “What about when”.

That’s not what whataboutism is; it requires the “what about when” to be followed by a change of topic, to distract the other party from the original topic.

Like when I made a comment on general etiquette and someone else shifted the conversation to one of the worst moments of the civil rights movement?

Nope.

"Whataboutism" is just asking you to validate your claims, I guess.

You don’t need me to validate it for you, search “never discuss politics or religion in polite company” - it was a common maxim.