Been reading the first article again and maybe this part is the key:
"Tempo will reference the rate at which a player manages to spend their full mana bar in order to gain minions which are purely focused on gaining control of the board."
This means that my earlier conclusion would be true, i.e. "removing the opponent's minion is NOT an increase in tempo, while adding a minion of my own IS"....
But even so I would still be confused, because that would also mean that both AoE-destroying multiple minions or killing a minion with your weapon actually aren't tempo gains - still, these are both identified as tempo gains in the article above.
Sorry guys, really don't mean to be a stickler for details but I really want to nail my understanding of these concepts!!
It's primarily just a wording type of thing. When doing the comparisons in these later sections, you have to ask "Gaining tempo relative to what?" Basically, purely for purposes of creating these tables, I considered, "Average" tempo to be a situation where you spend most of your mana each turn towards advancing the board. To understand the reason why, imagine if it's turn 10 and all you did was play a Knife Juggler. This is technically a tempo increase, since it's something you have on the board that you did not before; however, from a practical point of view, your opponent is almost certainly going to play more than 2 mana on the next turn, so your tempo is going to fall behind them. As such, when creating the tables, I had to define a baseline situation, and I ended up choosing a situation where you spend almost all your mana.
As for the spell, what often happens with direct damage spells is that you kill a minion of roughly comparable cost to the spell. So for example, if you use a Shadowbolt to kill a Raging Worgen, the effect is more or less tempo neutral, since you spent 3 mana to remove 3 mana of value from the table. This may vary a little bit, and obviously if you can use a 2-mana spell to remove an 8-cost minion that's at full health, that will be a tempo increase for you; that's not the normal situation for removal spells however.