If he's 70 and running 19 flat in races, yes, that's world class. If he's 68 and running 19:59, that's really impressive but maybe not quite world class. For comparison, at last year's USATF masters 5K championships (in Atlanta, so probably a hilly course), the top three men's 70-74 times were 19:30, 19:43, and 20:19. The top five men's 65-69 times were 17:53, 18:24, 18:55, 19:54, and 20:09, though the first four were all ages 66 or 67. If his Strava times are on training runs as opposed to races, it seems likely that he could go faster in races and so is in fact world class. I suppose you don't want to give his name, but if you say what city he lives in (unless it's NYC or LA), I expect someone in the over-50 thread would know who he is, as there can't be that many old guys in the country who are that fast.