I don't use gmail so I'm surprised at how low the (useful) information density is, in either version.

Extrapolating from that screenshot, it looks like that window could hold about a dozen emails. I use the no-frills Mail.app on macOS and a window that's takes about half or two thirds of my screen, and fits 40 emails. In my case I also get to see the "To" address in the list, helping me quickly triage the emails that are not sent to me alone.

Do people really prefer low density interfaces these days?

Removing the visual clutter is certainly an improvement but desktop apps have the menu bar where they can put all the actions, whereas a web app is more limited: it's either a mess of buttons or hamburger buttons which often end up a bit clumsy.