I believe if you look at their genetics they fit snugly within the tree of life
https://www.onezoom.org/life.html/@=111445?img=best_any&anim...
(hopefully that links to the right place)
There’s some speculation that a large amount of their genetics were transferred from other animals.[1] I wonder how much this muddles the tracing through the evolutionary tree.
If I recall correctly, the tardigrades (and extremophiles like D. radiodurans) have evolved to handle damage brought on by desiccation. As a fortunate side-effect, this general robustness also protects against radiation.