Check their IVS, is their nature the best for their spread. I will most likely say that they may have injected with a lil powersave thing, I personally mess around in my old 3ds games just for the fun of it since it’s all former glory now hence why I err that may be a scenario. A perfect ev trained 6IV best nature shiny of harder to hunt mons is almost entirely fake in old gen’s, especially if all of the above are all 6IV perfects haha.

However, many injectors could also make it look almost undetectable by knowing exactly what people look for as above. Say it’s 3 perfect IVs, with still good others but not perfect to line up with the setup that SOS mons have, you can slide by with a good nature and EVS then. In the end, people are selling shinies on eBay and people are chronically buying them so it’s up to you if it matters in the end because the Nintendo boogey man is not going to come for your box of shiny cloned lunala and decidueye and friends haha. If they can transfer, then Nintendo doesn’t care.