It depends on how they define "wilderness." I'm currently living in The Woodlands, TX (actual name) which people from Houston consider "the country", but there is an average population density of 2,500 people per square mile. The developers have just gotten really good at hiding it: things like strips of trees that border the main roads, blocking the view of the suburban sprawl. Roads that curve pointlessly so that you can't see down the length of them. When you drive through this area you get the impression that it's still somewhat natural land... until you take notice of the long, long lines of cars everywhere. Or the 4-story apartment complexes. Or you take a turn off any main road and get lost in suburbs for days. Or you look at satellite photos from 10 years ago, and compare them to recent photos... that lays it plain. You can hide this stuff from earth-bound humans' line-of-sight pretty well, but not from an aerial photo.
I haven't studied biology in detail but I don't think you need to hit 2500/mi population density for there to be severe disruptions to an ecosystem. I get the feeling the wilderness loss they're talking about isn't "small town becoming big city", it's "undeveloped land becoming partly developed land". Think of light pollution - the light bulbs themselves take up a minuscule amount of space, but their pollution covers the majority of the planet, already, today.
My personal definition of wilderness is being able to walk for a whole day in any direction without seeing a sign of human activity. The only time I experienced that was in rural Finland almost two decades ago.