Shoulder facing is a large part of reading. Slants have a shoulder facing the BBack. If the DT comes straight through the shoulders are more towards where the QB will be on a keep.
I could be wrong. How can he make the right read and still get tackled. It happens a lot on the triple on the give. The DE plays a middle ground. This allows him to read the QB. If he does this slow reading, a give will get 3 to 5 yards. But if the QB keeps then the DE get a 1 yard loss. I believe the QB is told to give in these cases.
The DT is not going as fast as expected. The fake mesh/bad mesh/no mesh is clearly visible to the DE. He sees the BBack does not have the ball and goes for the QB. The DT's shoulders scream BBack, but he does not commit. A good mesh would have forced him to commit.
This is not a guess who he is going to take. There are keys to read and good technique to follow.
What keys do you see that says he is going for the QB. I'll give you this... longer mesh force DT contact with the BB to get to the QB (as the BBack is in between the DT and QB.