I agree with that, the occasional skin fluke doesnt necessarily warrant treatment.
Its very unusual for koi to die from flukes, unless a massive infestation, so chances are there is also something else going on. Flukes and costia seem to often go together- so is definitely having a close look for them at x200 and x400 on another fish ir two.
Scrapes and microscope skills just take practice really so the more you do the better you will get.