It probably is that the installer of the spark plug did over tightened it, but it doesn't mean they used an impact gun. There may be other things going on here which has seized the plug in the hole or is making it very hard to turn.
First of all, there are recommendations for plug tightening. The following chart from this website shows:
The same site states:
Tapered-seat spark plugs should be installed by hand until finger-tight. Then use an additional 1/16th turn applied with a spark plug wrench to ensure the spark plug has been tightened to the manufacturer’s recommendation for a 14mm thread. Alternatively, using a torque wrench, tighten the spark plug to approximately 7 to 15 lb. ft. for 14mm threads and 15 to 20 lb. ft. for 18mm threads to ensure a gastight seal. Overtightening a spark plug will stretch the plug body and potentially result in breakage upon removal.
For gasket-type plugs, install the plug until finger-tight. Then if the plug is being installed with a new gasket, use a spark plug wrench to apply a 3/8 to 5/8 turn – depending on head material. (See chart below.) When reinstalling a used spark plug with a used gasket, only ¼ turn is necessary, regardless of head material.
With that said, it's easy to see how a spark plug could get over tightened if the installer isn't paying attention to what they are doing. Also, if the installer didn't ensure the threads were clean of carbon, this might exacerbate the situation. Would over tightening be the only thing goin on here? I don't believe so.
What is really going on here is probably two fold.
First, the plug being over tightened, for sure. There is the possibility the installer has tightened it tight enough to deform the threads of either the spark plug or the hole, making it much harder to remove.
Secondly, depending on how long it's been since the plug was changed in the first place, there may be a bit of "space welding" going on. This would be where two dissimilar metals have some galvanic corrosion may have taken place, which has caused the threads of the hole and the spark plug to seize together, making it near impossible to remove. This is why I always give a strong recommendation to use anti-seize on plug threads to help prevent just this type of issue. anti-seize also helps to seal the threads and to increase electrical continuity between the plug and the head, so is a win-win all the way around.
The situation with your bike is ugly, no doubt. I don't believe any mechanic in their right minds would use an impact to tighten a spark plug, if they could even get the spark plug to survive such an event. It is very easy to over tighten them, though. A 3/8" drive ratchet provides more than enough torque to get the job done, yet unwitting mechanics will use 1/2" breaker bars and man handle things into place. Very ugly.