Whataboutism would be if I said “Biden’s support for the Gaza genocide is acceptable because Trump rapes little girls”. What I actually said was “Biden’s warmongering is nowhere near as severe as Trump’s warmongering”.

That's complete bullshit. There's no requirement that you change the topic of the thing in question, that's 100% arbitrary.

If someone criticized China for saber-rattling with Taiwan, and in response I brought up that the US had not only saber-rattled but actually conducted full scale invasions in Iraq and Afghanistan, would that be a whataboutism? Because I would certainly expect to see people lob the accusation at me.

because the context was a tweet that says “If you vote for Biden, you will be at war, so you should vote for Trump”.

The context is a comment thread where someone said that the first parts of the tweet were correct, but not the line endorsing Trump. You're being very dishonest by ignoring that. If you want to make a comment about the tweet in full, then make a top-level comment.