Aren't gym leaders supposed to be able to deal with any Pokemon type they're facing?
The users complaing about the absence of helpers do have a point here: If such a major crisis occurs, you'd expect at least half of the gym leaders and the Elite Four to be there to help. But there was nobody there except those three helpless guys.
So, yes I'd call this a major weakness of the episode (fortunately the only one).
The fact it's in Hoenn really shows this. If it were in Kanto or Ash and Go were already in the area when they discovered the sandstorm I'd give it a pass, but this made the news in Kanto and Professor Cerise okayed two ten year olds to investigate. Assuming at least a day for Ash and Go to get to Hoenn, plus getting from Slateport to outside Mauville, you'd expect some kind of official response to stop people going in to be established. The timing on the crisis is completely off. It's possible they caught a flight considering the urgency, but that still doesn't account for the lack of on the ground support.