The statistic I cite is not in the slightest bit misleading. It is supremely illuminating. For roughly 90 years, the overall BA among all players has tended around .262, with the standard deviation around that number shrinking steadily. This reflects what you would expect in any system of competition that evolves by conferring success upon those who are bigger, faster, stronger and smarter. If the system is limited as to pool of available talent, then the biggest, fastest, strongest and smartest will dominate the competition because the competition is statistically unlikely to present comparable specimens. The best are able to destroy the competition. Open the doors to more candidates, and suddenly there are a lot more big, fast, strong, smart candidates. That fosters sharper competition, with a tightening of the difference between best and worst. In this respect you are absolutely correct to observe that had the pool of available candidates been allowed to play MLB in the early days of baseball, then it is likely that the standard deviation around the overall BA among all players would be lower. But that simply proves the point: with a greater pool of talent comes increased overall excellence among all players. Better pitchers, better hitters, better fielders.
The advent of bigger gloves, superior fields, tighter balls and the like does not change the significance of the above. It actually amplifies it. Bigger gloves give the advantage to the defense. Superior fields (carefully groomed infields and the like) can cut in both directions--greater predictability on balls that are hit and better footing (favoring the D), sharper and faster bounce (perhaps favoring D, perhaps not). Shorter fences allows for more HRs. But it also keeps the ball closer to home when it doesn't clear the fence-- one reason why no one will ever hit 36 triples in one season, as did Chief Wilson in 1912. Lower mound helps the hitter. But then, today's approach to pitching includes higher velocity and greater variety in pitch type; meanwhile, hitters rarely see the same pitcher more than 3 times in a game, often not even that. Managers bring in specialty pitchers for specific occasions. Teams track hitters' tendencies, preferences and strike zone success. Fielders regularly position themselves in response to each hitter in a way that they rarely did 60 years ago. A hitter may see two lefties and three righties in a single game. Today's fielders are also remarkably acrobatic compared to the players of 75 years ago.
So, in this respect, there is no question that hitting .310 today can be understood as every bit as impressive (if not more so) as hitting .350 in the 1920s. Think about it. George Sisler hit .407 in 1920. Are you saying that if a player today hit .407 that would reflect a comparable achievement? Even if Sisler were able to travel in time to 2018, and was given every advantage while growing up to be groomed as an MLB player, can anyone really believe that he would ever hit .407 (or anywhere near that) playing in the majors today?