You can see Sainz drifting over, Perez maintains his line. Neither are innocent, but sainz is more to blame
But Sainz was taking the normal racing line and Perez was the one who had forward visibility of the other car and had the opportunity to react, but he didn’t. Sainz never made a sudden move, and depending on which angle you watch the incident from it looks like either they both drift equally into each other or Perez is just napping and lets Sainz drift into him—it’s not as if drivers don’t squeeze each other on straights, so Perez really shouldn’t have been caught out there. The 50/50 call from the stewards seems pretty reasonable to me.
Sainz’s real mistake was in thinking he could have a pop at Leclerc: that move was never on and was only going to push him back towards Perez. So the last few hundred yards were certainly not the smartest bit of driving from Sainz, but I don’t see him as having significantly greater responsibility for the crash.
Great drive from Piastri though—another ruthless pass, and a supreme defence. Thought Leclerc’s defence from Perez at turn 1 was pretty stunning too. And a cracking job from Colapinto, emphatically vindicating Vowles’ decision in no time.