You can pinch-zoom on mobile, but only while you're holding it down. I suspect again this is intentional and the two go together - they don't want you zooming in on limited resolution images, that'll look even worse.
This is a relatively recent feature. And yes, it is almost certainly because the images are awful quality before zooming in, so they want to limit how obvious this is.
The one and only supported image resolution is also lower than any of the screens I use.
The app is really hard to believe from a technical perspective, in general.