The response you got on Garmin's forum is correct; the Nuvi cannot use "custom maps" as Garmin defines them. But that term is ambiguous. When Garmin says Custom Map they are talking about .kmz files containing raster imagery, such as a scan of a paper map or satellite photos.
Before Garmin introduced this feature, others used the term "custom map" to describe any map that you make yourself (as opposed to a Garmin map that you purchase). This site contributes to that confusion because it still uses "custom maps" in that context, for example: http://www.gpsfiledepot.com/maps/
You can certainly use maps you make yourself on the Nuvi, but some features may not display correctly. But these maps are more difficult to make than the Garmin "custom map" .kmz files.
When you zoom way out on the Nuvi, you will see shaded terrain and I think that's what you are describing. This is not really a "topo map", it is just shaded terrain that comes from the very low resolution basemap on the Nuvi. You only see it when zoomed way out because it would look terrible (all pixellated and blocky) if you looked more closely. But this shading is different from the raster imagery in a custom map. It is digital data that defines the height of the terrain (known as DEM data). The nuvi interprets that data and displays it as colored/shaded terrain. That is not the same as actually showing an image.
This feature came to the Nuvi series a number of years ago. Here's an old thread with screenshots I posted back in 2008: http://www.gpspassion.com/forumsen/topic.asp?TOPIC_ID=113310