What do top's %MEM and VSZ mean?

The man page you refer to comes from the procps version of top.

But you're on an embedded system, so you have the busybox version of top.

It looks like busybox top calculates %MEM as VSZ/MemTotal instead of RSS/MemTotal.

The latest version of busybox calls that column %VSZ to avoid some confusion. commit log

It is likely that rstpd has a lot of dynamic libraries linked to it, but they have not been loaded into memory yet.

See Why the value of VSIZE in top is different from the value of VSZ (Virtual set size) in ps?

What does ps aux show for that process?