There is a difference between a creature actually attacking, and being declared attacking. The same is also true with creatures actually blocking, and being declared blocking. This arises from the fact that while not explicitly a status per the official rules, ‘attacking’ and ‘blocking’ function in game terms almost identically to a status.
- Abilities that trigger when a creature (or player) attacks or blocks only trigger when a creature that was already in play becomes an attacking or blocking creature (that is, they trigger on the state transition into a state of ‘attacking’ or ‘blocking’). This means, for example, that if you attack an opponent with Kaalia of the Vast and use her triggered ability to put Aurelia the Warleader into play tapped and attacking, Aurelia’s triggered ability that triggers when she attacks will not trigger that turn.
- Costs associated with attacking or blocking are also evaluated in the same way. This means for example that someone who is attacking an opponent with an animated Den of the Bugbear when that opponent controls a Ghostly Prison only needs to pay for the Den of the Bugbear that they are attacking with, not the Goblin token that gets created.
The second point is what is relevant here. Tapping a creature is part of the standard cost of attacking with that creature, so it gets ignored if some alternate cost is paid, or if something completely circumvents the cost (as is the case with any effect that puts a creature or creature token into play attacking). This is actually why cards that put creatures into play attacking also put them into play tapped, because that provides consistency with how the rest of the game works without making things even more complicated.
This particular aspect of attacking and blocking also has a third implication, namely that abilities or spells that say that a creature is attacking or blocking ignore costs associated with the action of attacking or blocking. This is why the ‘Goaded’ keyword, as well as creatures like Juggernaut explicitly state ‘if able’ in their effects. Without that, such effects would allow you to circumvent additional costs imposed on attacking or blocking (such as by the aforementioned Ghostly Prison).