I suspect that you said it out of ignorance about what is "doable" with a factory Ruger rimfire, a brand, incidentally, that doesn't have a reputation for match rifle accuracy. (Please note that here "ignorance" means simply "not knowing," not the mistaken view that somehow it means "rude.") The claim may have been compounded by ignorance of how to interpret groups on a target. Perhaps you know this, but is important to keep in mind when discussing group size on a target, each round fired, each hole on the target counts. There are no mulligans for "flyers," no saying "I knew this shot would be off -- I called it -- so it doesn't count. A ten shot group that is 0.5 MOA at 100 yards means that all ten shots count and all ten must be 0.5 MOA (or very close to 0.5"). There is no shame in conceding that you didn't know this; a lot of people who may be relatively new to shooting wouldn't.
As a final observation, there must be some ignorance here because if a shooter shot a ten shot group at 100 yards with a rimfire, be it a match rifle or a more run of the mill model, at 100 yards, he would know that such results would be worthy of a picture for posterity. Such results don't occur very often indeed.
