Yeah this isn’t super clear or logical, which is why this is one of the events that I want to revisit in 1.3.
The grunts are after the Lucario because there are rumors that it Mega Evolves. Tormenting the Riolu is just meant to bring out the mother. This isn’t super obvious from the dialogue which goes from talking to the Riolu to immediately saying:
This is a bit ambiguous because it’s not exactly clear if the grunt is referring to catching Riolu before Lucario Mega Evolves or Lucario before Lucario Mega Evolves. But it is meant to be the Lucario.
Where this part really doesn’t make much sense though is that there isn’t any indication that Audrey hasn’t already figured out how to Mega Evolve Pokemon yet or that her Crawdaunt isn’t able to Mega Evolve by that point as this event just disappears from recollection until a brief nod to it in the post-game. So unless the purpose of this event was for the cultists to try and figure out how to make their Pokemon Mega Evolve without a Mega Ring, there is no clear reason for any of this occurring.