NavList:

I think the first question to be asked is what sort of "electronic charts" were they using? 

Let's remember that commercial vessels are now allowed to use electronic charts and are not required to have paper charts aboard.   But these are "professional grade" electronic charts, published by national charting authorities like NOAA.

Recreational sailors have two choices -- "chartplotters" that use data produced by independent third parties and computer (typically PC) - based programs, some of which can read the professional "ENC" format charts.

It's not clear which sort of "chart display" the boat was using.  My limited experience with ENC-format chart displays tells me that they would likely display an important navigational hazard regardless of the zoom level of the display.

I teach navigation at all levels for the US Power Squadrons and I always caution my students about chartplotters because of the third-party data cartridges they rely on.  

First of all, I have never heard how the cartridge producers check the accuracy of their data.   For example, do they do something as simple as verifying their aids-to-navigation data against the Light List??  I have visions of poorly paid third-world workers who are absolutely ignorant of navigation "digitizing" NOAA paper charts.

Secondly, I have seen cartridges with dangerously incomplete data.  Right before I moved from the Boston area to the SF area 20 years ago, I visited the Newport boat show one last time.  Several electronics manufacturers were displaying their latest and greatest chartplotters (as I recollect, it was when they were first becoming available with color displays).  I fiddled with one, scrolling from Newport to nearby Cuttyhunk Island, a popular anchorage for recreational boaters.  Cuttyhunk is easy to approach, but there are significant reefs sticking out into the two most common approach paths.   The ends of these reefs are well-buoyed.   But neither the reefs nor the buoys showed up in the chartplotter!    I immediately found the most senior-looking person in the booth and told him that someone could buy one of his chartplotters, take it to his boat, install it temporarily, sail over to Cuttyhunk for the night -- and rip the bottom out of his boat!  The gentlemen simply shrugged and said "we don't make the data cartridges."