I would say the reason for that is that the Gardevoir is faster, traced the Sand stream and thus set up its own, not-smooth rock boosted sand before TTar could.
I am well aware that trace makes garde faster. Here's the evidence that it was not the case. (did you even watch the replay)
1. The sandstorm subsided, not only was garde faster
2. Garde did not even have trace
Turn 5
Mad At Myself: get read like a fuckin book
★LT1A BLUNDER: i played this guy 10 times in a row yday
★LT1A BLUNDER: relax
Bacon and Blissey withdrew My little power(Keldeo-Resolute)!
Bacon and Blissey sent out WAYAUUU (Excadrill)!
Latios, come back!
Go! Gardevoir (Gardevoir-Mega)!
Pointed stones dug into Gardevoir!
The sandstorm subsided.
And yes,
I did have the smooth rock