its not even close to a fugue! the only "complex" thing going on is the part that is harmonized...
The fugue isn't a complex form, it's just a way of structuring improvisation. You take a melody, break it into its constituent parts, then elaborate it harmonically in what, in essence, is a series of call-and-response segments. It's not supposed to be radically complex, it's supposed to give an organist a means of improvising a prelude on the spot before the start of the worship service.