You do, in fact, actually redo character models and other stuff for a beta release. This is early alpha, they're using placeholder models and textures, probably from previous Tribes games, nailing the core gameplay loop before working on graphics.

This is standard practice.

Yes it is.

The last Tribes game before this was Ascend, released over a decade ago.

Polygon targets, textured sizes, shader technology and much else has evolved dramatically since then.

You may reuse old assets as placeholders during development (but this can be problematic for the same reasons why temp music in filmmaking is problematic), but you absolutely do not have assets already made. Assets change over the course of development, often right up until release.

I can't speak to whether or not these assets are from Ascend or earlier, nor can I speak to the visual production quality Prophecy is capable of, but seeing shoddy looking visuals from a pre-alpha title is normal.

Source: I am a trained game developer.