If masturbate can reduce stress, could it also help in anxiety treatment?

Not so much: Masturbation will not produce lasting positive effects on anxiety problems. It's better to address what is sustaining the anxiety problem and to intervene there. Often, anxiety problems persist due to repetitive dysfunctional or unnecessary thoughts. Cogntive-behavioral techniques can be used to revise these thoughts/behaviors to relieve anxiety.

Nice try: While it does relieve certain stresses, it is not a substitute for appropriate psychological services as recommended by a clinician.

Different processes: While often used interchangeably, stress (distress & eustress) is a natural response to internal and external stimuli/pressures and anxiety is more associated with the "fight or flight" mechanism and threats to one's personal safety. Masturbation may help temporarily reduce physical and psychological pressure, but it will not likely enhance one's sense of being safe.