“After directly using an ability that costs 20 or more mana, your minions gain increased attack speed for a short duration.”
The exact duration needs to be specified. There should also be an “[alt] more information” section about if and how the mana cost interacts with channel cost.
Given that many other nodes in Forge Guard skills, like Ring of Shields have a number of ‘if used within 4 seconds’ triggers, I’m going to suggest Hammer and Anvil is as well. H&A is a pretty old skill so it’s likely this just got missed in some of the update tool tip passes they’ve done.
They should file an ingame Bug Report so it’s specifically on EHGs radar.