Afaik it should see both inputs simultaneously if you have n-key rollover, which is a pretty common feature now. I think the last time I was looking for a keyboard, they all advertised n-key rollover. As such, it’d be on the game, not the OS.
You can actually test this in the Unity editor if you have some programming experience (or I assume any other engine for that matter). It’s very easy to write a character controller in which A+D results in the game using the last input instead of adding the inputs together. I actually had to learn how to make a game not do that. It wouldn’t function exactly like the keyboards currently under fire, but it would function very similarly.
I’m pretty sure you can also do this very easily with a macro (assuming you have a keyboard with n-key rollover), no special keyboard required as well.
Edit: your specific example is an example of a cheap keyboard losing inputs, which n-key rollover fixes. Ironically, find the right cheap keyboard and you might be able to partially reproduce the effect they’re talking about (having one key override another other).