Ok, I managed to find the slide I was thinking about. There was a slight misremembering. Here is the bullet point that is important: "Even with fixed number of elements, queues with variable-sized data are still variable-sized (also true of similar mechanisms such as Network Streams or Shared Variables)".
So the point there was that a queue of strings or arrays will not be preallocated since it is variable sized data even in a fixed sized queue. With the RT FIFO, you can actually set the number of elements in the array, so it can preallocate.