Being a runner at 3d base your first step should be back to the bag if the ball is in hit in the air or at least freeze. Once it gets over the infield your first thought is to tag up. If it drops in you will score easy and if it is deep enough you tag on the sac fly and score. Now in this situation if he tags as he should be doing (unless it was extremely shallow like right over the SS) and seeing that the guy was diving there is going to be a chance he makes the catch and if he does as the OF did, Poirrier should have scored easily even if it is shallow because the guy had to dive. There is no way even if shallow the guy makes a diving catch and comes up to throw a tagging runner out at home.
Maybe the ball was really shallow like a pop up just over the SS's head and someone who watched the game (BOP) and fill us in. Maybe Poirrier did not show good instincts of watching the ball flight and he did not think he OF was going to have to dive, but having good instincts is a huge part of being a good base runner not just having good speed.
Sounds like bad base running to me!