Tempo is basically frequently contrasted with card advantage

I've never heard that.

In fact the whole reason that SI agent is a good card is because it is removal that has a body, which generates card advantage while it advances your board while hurting the enemy's. Tempo.

Backstab is absolutely a tempo card, and it's called that because it generates tempo.

Tautology. It's a tempo card because it generates tempo?

Backstab + evisc is not inherently good tempo just because it's removal.

Tempo is a complex set of ideas combining board states, cards in hand, etc, culminating in "momentum" of the game. It's more than just kill spells.