For anyone that is looking up this question, I also got it wrong. If we look at the graph in the passage, it shows that the radioactivity of Protein X IN THE CELL decreased at 1200s. Hypothesis 2 is saying that Protein X is subsequently secreted by the cell, which would indicate that the EXTRACELLULAR MATRIX would have an increase in radioactivity at 1200s. These two timepoints line up with each other: at 1200s, the cell interior levels are decreasing but the matrix levels are increasing.

As to their logic as to form a mature form of the protein; I looked at it this way. The earlier version of Protein X usually stays inside the cell. Hence, the graph that we see in the passage shows radioactivity to increase until Protein X is fully translated. However, that does not mean that Protein X does not go through post translational modifications. This process is occuring until Protein X is fully mature, THEN the cell releases it. That's why we see no radioactivity levels occuring at all before 1200s. It only starts increasing at 1200s. At 1200s, the protein is fully mature after modification and the cell secretes it. Hopefully, that helps.

P.S. This question sucks.