Is this a suburb?

[Verily]

I'd have to see a picture of it to really get the full picture, but a population density of nearly 22,000 per square kilometer makes it kind of unlikely.  I don't think that would even be possible with single-family housing.

Guttenberg consists of tightly packed multi-family homes and apartment buildings, many with commercial developments on the first floor. It is only four blocks wide and around 15 blocks long and consists of rectangular city blocks as one might find in any large city. It is overwhelmingly Hispanic, and most of the rest of the population is Italian.

[Verily]

It's the most densely populated municipality in North America.  I highly doubt it qualifies as suburban.

Guttenberg is purely a dormitory town, and as such could not of course be anything but a suburb. (That's part of the reason for the pop. density - no commercial or recreational areas to speak of.) Population densities and voting patterns and income levels have nowt to do with it, really.

But Guttenberg's commercial areas are right next door in West New York. You're looking at each municipality in isolation, which is a stupid way of doing things. There are much larger areas in Queens, for example, that are purely residential and identical to Guttenberg, yet you would call them urban because of arbitrary lines on the map combining them with the rest of New York City. Guttenberg is an urban residential area; the fact that it was made independent in the early 20th century doesn't change that.