Every other site that I've come across that uses a carousel seems to handle not messing with my browser keyboard shortcuts just fine.

Alt + Left-Arrow is not the same key press as 'left and right arrows'.

Yeah, it seems mildly inconsistent. I think it's not intending to pick up Alt-Left, but it does sometimes.

In terms of events, isn't Alt + Left-Arrow equivalent to left arrow if they don't consider the 'Alt' flag on the keyevent?

It's completely undiscoverable, and also easy to hit by accident.

I'm not sure what the value is, either, since you don't know in advance what article you're switching to.

I have a number of tics (if any of them were vocal, it would be enough for a Tourette's diagnosis), including that I mash the left and right arrows very frequently.

That made reading this article very difficult. It's an accessibility problem.