Yes, the strap is compatible with the fenix 5x. Since a chest strap can have ekg accuracy, it is much more precise than the optical measurement on the wrist. Therefore, the chest strap is recommended especially for sporting activities. Depending on the type of sport, however, a different version would be suitable to take full advantage of the functions of the fenix. hrm run and hrm tri, for example, can also record additional running data, and hrm swim and hrm tri work under water.
I hope this helps you.
Kind regards