Further, any traffic that doesn't fit the statistical properties of most other users is going to be an easy outlier to spot, and VPN users will tend to cluster

Further, any traffic that doesn't fit the statistical properties of most other users is going to be an easy outlier to spot, and VPN users will tend to cluster with each other. E.g., if there's no traffic to any search engine or social media then VPN use is statistically more likely (or it's a point of sale device, etc.)