Most likely the mass-uncroak trick that he wants Wanda to do here is something that can only be done on your turn, like Veiling -- it counts as casting a proper 'spell', and as we were told with that, you can't cast outside your turn.
But you can still use magic outside your turn, obviously -- combat things like Shockamancy, or the EyeBooks and table, or the Archons' death rays, or triggering the air defenses. My guess would be that uncroaking individual units one by one by hand is something like that -- Wanda has it as a 'special attack' or something rather than as a proper spell.
At least, I've seen several games with mechanics that work something like that.