The reason the 67s were more impossible than the 21s is because they had losing records the previous eight years, including 9th the last two, and 8th, 7th and 8th right before that. The 21s had eight World Series champions from three years before, and 10 guys with rings. The 67s had Elston Howard for two months.
However, the 1967 Red Sox also had the best player in baseball and almost history; only one guy all-time ever had a higher single-season WAR than Yaz' 12.4 (ironically, that man was once a lefty pitcher who won three World Series for Boston). The 67s also had the AL Cy Young winner, Jim Lonborg. Yaz and Lonnie were pretty good before '67, but the timing of both erupting to dominate both sides of the ball in the same year on the same team made their season more improbable than anything.
Despite all that evidence, the Red Sox won 24 games a year ago... while the guys wearing the current laundry finished yesterday with a total of 98 wins. That's just impeccable.