There's nothing wrong with normal Garchomp in BH in general. Yes, its attack power is significantly lower, but its 102 speed lets it break two important speed tiers that Mega-Garchomp falls short on. 95 and 100, specifically, which is where many of the major Pokemon reside (as well as 90). Of course, you need to be running a set that leverages its higher natural speed, which is something that Garchomp wasn't doing, so in this case it was a bit dumb.
...though I'd argue him using Zard-Y without Defog or Spin support is far sillier than anything else he did.