Possible problem with the Pseudodragon errata

Quote from Wolfenbach >>

That would be like saying someone casting Fireball on top of you is not an attack, since they dont make an attack roll.

Yeah, Fireball isn't an attack, for precisely that reason. While this is no longer stated as explicitly as it was in the 5e PHB (the quote below), it's still the case in the 5.5e rules as well.

If there’s ever any question whether something you’re doing counts as an attack, the rule is simple: if you’re making an attack roll, you’re making an attack.
https://www.dndbeyond.com/sources/dnd/phb-2014/combat

(The Unarmed Strike's Grapple and Shove options are exceptions to this, since the Unarmed Strike's own rules define it as an attack.)

Quote from Wolfenbach >>

These are all of the "Actions" anything can do in D&D 5e 2024:
Attack, Dash, Disengage, Dodge, Escape, Help, Hide, Influence, Magic, Ready, Search, Study, Utilize.

 Those are all of the Actions every creature can take. That's not all the actions any particular creature can take.

On your turn, you can take one action. Choose which action to take from those below or from the special actions provided by your features.
https://www.dndbeyond.com/sources/dnd/phb-2024/rules-glossary#Action

A monster can take the actions in [its statblock] or take one of the actions available to all creatures, as described in the Player’s Handbook.
https://www.dndbeyond.com/sources/dnd/mm-2024/how-to-use-a-monster#Actions

 
In the Psuedodragon's case, they are indeed taking the Sting action.