en.wikipedia.org/wiki/Mulholland_Highway

This sentence appears to conflate Mulholland Highway with the broader Mulholland Scenic Parkway and Corridor.

Official park-agency descriptions draw a clear distinction:

• The Mountains Recreation and Conservation Authority says the "fifty five-mile Mulholland Scenic Parkway and Corridor" includes Mulholland Drive in Los Angeles, while "Mulholland Highway starts in the City of Calabasas and twists through the Santa Monica Mountains for thirty miles to Leo Carrillo State Beach."
• A National Park Service planning document similarly says the 55-mile corridor includes Mulholland Highway in its western portion and Mulholland Drive in its eastern portion, and that Mulholland Highway runs "for thirty miles to Leo Carrillo State Park."

So a 50-mile figure is not supported for Mulholland Highway itself; it overstates the road's length by mixing it up with the larger scenic corridor that includes both roads.

Official sources distinguish the eastern and western parts of the Mulholland corridor by different road names.

• The National Park Service says the 55-mile Mulholland Scenic Parkway and Corridor includes "Mulholland Highway in its western portion" and "Mulholland Drive in the eastern portion of the corridor."
• The MRCA likewise says Mulholland Drive starts west of the 101 Freeway in Hollywood, while Mulholland Highway starts separately in Calabasas.
• For the Hollywood area specifically, the MRCA's Jerome C. Daniel Overlook page gives the address "7036 Mulholland Drive, Hollywood" and says that from there the Hollywood Sign and Griffith Park Observatory are clearly visible.

So the article's Hollywoodland/Griffith Park sentence misnames that eastern segment as Mulholland Highway. Officially, that stretch is Mulholland Drive.