There are plenty of alternative airports in Hawaii, but none in-between of course. So what the author is getting at is that you're probably going to be dead if you run into trouble.
While I can't say I know anything much about the Cessna 402, it apparently has a normal range of around 1,273 nm (1,467 mi, 2,360 km), compared to a distance Hawaii-Oakland of 2050nm (2359mi, 3797km). Hence, you're going to have to install additional fuel tanks and fly pretty heavy.
There a few considerations if you get a problem:
- You will have to descend since the engine can't generate enough thrust at that cruise altitude. I found a source stating the ceiling for both engines is around 26180 ft compared to 11320 ft for one engine.
- The fuel consumption increases in the remaining engine and will be greater than the total for both engines for the same flying conditions.
- You're going to have to run the remaining engine hard for the remain of the flight.
- If your temporary ferry tanks screw up (or any of the normal ones for that matter as well), you won't make it.
- You may damage/break the gear if you have to return to the airport overloaded. "I once saw a Cessna 402 parked on the ramp at Honolulu with the gear punched half way up into the wings from a poorly executed overweight landing." Source
I can't do a calculation on for what portion of the flight you would be 'screwed' but I imagine it might very well be a bit of the journey. Several people seem to be in the opinion it's a pain to fly heavy and difficult to with one engine.