Timing and scale. 800 Minuteman I missiles (the 1962 model were produced), each with its own guidance computer.

The Apollo Guidance Computer came out in 1966, by which time transistorized computers were a mature technology. The IBM 1401, all transistor, came out in 1959. IBM System/360 systems came out in 1964.

I don’t think it’s transistors that the article is alluding to when it says “semiconductors” but integrated circuits. While individual germanium transistors were (as you note) already widely used, the AGC was the first computer to make use of integrated circuits, in which multiple transistors are etched onto a single piece of silicon.