1. We're alone (or very nearly so) due to an early filter. i.e. it was vanishingly unlikely for life to develop as far as it has here.
2. We're alone (or very nearly so) due to a late filter. i.e. life develops in lots of places but, terrifyingly, it becomes overwhelmingly likely to wipe itself out rather than spread.
3. Life develops frequently but there's some sort of interstellar ecology that makes species stay hidden or be wiped out.
4. Life develops frequently but is overwhelmingly likely to develop some other interest (becoming inward-looking, or exploring other dimensions) rather than colonising all of space.
This article's suggestion that Earth may be in a backwater wouldn't be sufficient by itself, so it is coupled with the idea, derived from observations on human colonisation of Micronesia, that expansion happens in waves and life on Earth happens to be in between waves. The problem is, advanced technological civilisations expanding across the galaxy isn't the same thing as Polynesian explorers eventually being followed by Europeans. Why would the waves die away? This is a late filter with extra steps.