Horizontal nail ridges always mean onychomycosis or are there other reasons for this?

Not necessary fungus: These may not be fungal in nature. There are other causes such as outside injury to the nail plate, or other systemic diseases.

Other reasons too: Horizontal nail ridges (or Beau’s lines )run across nail & might be signs of diabetes, thyroid disease, kidney disease. They may occur in people who had syphilis or muscles or who had chemotherapy. These should be evaluated.