how exactly is "All lives matter" a heated topic?

Really? Okay "Black lives matter" is a rallying cry against racism. The entire point is to bring attention to social inequalities in modern society. No one is arguing against the fact that "all lives matter" - because of course all lives matter.

Since you bring up gaming let's use that. It's like me saying "Mac gamers matter" and you counter with "No! All gamers matter". The point is that Mac users don't get access to as many games because of compatibility so we say "Mac gamers matter" to bring attention to that. In that context, wtf does "All gamers matter" even mean? PC users don't have any real issues when it comes to game availability, so what's the point of that statement other than simply being dismissive?