Looks like an indian club! I think the idea is similar to light kettlebells, gives you a bit of resistance without too much"
On other wrist strengthening techniques, quality of sensible S&C differs wildly in muay thai gyms especially in Thailand, but I have seen something similar to that in Chinese martial arts like Shua Jiao:
Because it's a wrestling style, they build up their arms with stone locks which are similar I suppose to kettlebells
For less 'martial artsy' ways of training your wrists, the best and probably most direct approach is to use things like grippers and dumbbells.
Hammer curls (at any weight) will target your forearms and wrist
You also have wrist curls which I'd only recommend doing with a light weight
Gennady Golovkin, who's possibly the hardest puncher in any sport p4p likes this:
He's got very strong hands (there are videos of him being able to hurt people's hand but squeezing) - and that strength in his wrist bulks up his already strong kinetic chain. As far as I'm aware he's never had a broken hand/wrist compared to some other fighters who always seem to have some sort of hand injury.