The Gulf of California is narrow. In a ship, you want as much room as possible between you and unknown shores. To prove California is not an island, you would need to explore the Gulf a long way (700 miles) to reach its northern limit.
On the safer west side, you don't know ahead of time where to expect the top of the island. If you travel 1500 miles north, you might just assume the top is still further north.
Even on the west side, no settlement north of the Gulf was established until 1769 -- just seven years before Baja was determined to be a peninsula.
Most of Baja in those latitudes is desert. It's hard to make a living there, so there's not much incentive to explore.
No, the article makes it sound like Spain knew before even 1600, but used these maps as a political maneuver.
> Spanish authorities and local residents were well aware where the actual northern terminus of the Gulf of California lay, but by extending the coastline north past Cape Mendocino and eventually even into Puget Sound, Sir Francis Drake's claim of Nova Albion for England (1579) could be invalidated by the priority of Cortes' claim (1533)
This can be seen by all of these maps of around this period that are made by Dutch/English/German cartographers that don't show California as an Island, including Munster's very popular one:
