The three teenagers overlooking the crime scene are one party, Donald Fouke and Eric Zelms are the second party, the third party we will get to in a minute or so. Donald Fouke is on camera on two occasions, stating he saw the Zodiac Killer motion towards a residence. In the 2007 Zodiac documentary that accompanied the David Fincher movie, he details the suspect heading up a stairwell to 3712 Jackson Street. See here. In an earlier documentary, he again mentions the suspect heading into the courtyard of a house on Jackson Street, stating "He came down the north side of the street, and turned and went up a flight of stairs into a courtyard." This of course makes a mockery of the Donald Fouke memorandum issued on November 12th 1969, that stated "The Zodiac Killer was observed walking in an easterly direction on Jackson Street and then turning north on Maple Street". No mention of turning into any house - and considering that he supposedly passed the Zodiac Killer in "5,10,15 seconds tops" - it is actually physically impossible for the killer to walk up the stairs to a residence, walk down and then travel to the end of Jackson Street from the aforementioned address in 15 seconds. So one story is fiction. Unless of course, the two police officers were parked up for a set duration of considerably longer than 15 seconds, which is something that Donald Fouke has always denied. But before we move on, note the observation in the memorandum was a final sighting of Zodiac, heading north on Maple and his other sightings were at residences on Jackson Street, neither of which mention the Zodiac running or wearing a grey jacket and trousers. In fact, he never mentions seeing Zodiac enter the park either.
The supposed sighting of the Zodiac Killer by Donald Fouke and Eric Zelms was at either 3712 Jackson Street, or a Zodiac walking east on Jackson, before turning north on Maple. Either way, Zodiac was somewhere just short of the Jackson and Maple intersection. So when the Zodiac described "disappearing into the park a block + a half away never to be seen again," this would place him at Spruce Street, the only clean thoroughfare to the Presidio Park he had so far passed, and the exact location of the eyewitness (or eyewitnesses) in the Chronicle article, thereby corroborating Zodiac's claim of entering the park where he said he did. The two closest residences overlooking the Julius Khan playground are 1 and 2 Spruce Street, so it's likely the witness was either a resident living at the north end of Spruce Street overlooking West Pacific Avenue and the playground location, a passing motorist or pedestrian, or for that matter, the unidentified 8-year-old witness, detailed in the FBI report in connection with Paul Stine's murder. But above all, we likely have a sixth or possibly more witnesses.