I think Will was actually giving the career "road" numbers for the two hitters...Joe D away from Yankee Stadium, Ted away from Fenway. The way it is phrased above implies that George was also removing DiMaggio's games at Fenway and Ted's games in the Bronx but:
(a) I doubt he had access to that data in 1987 or whenever he wrote the book...there were no references that I know of at that time that would have provided that information.
(b) Joe D's career road averages were .334/.406/.611. Ted's were .328/.467/.615. So the batting average and slugging were essentially the same, but Ted walked a lot more (or put differently, Ted made many fewer outs).
And of course Ted's career counting numbers are much more impressive...he never had a down season except for 1959 and kept putting up big numbers well past the age at which Joe D had retired. Both missed 3 years in WWII, so that's a wash at best, and Ted of course missed 2 more seasons due to military. Give Ted those five full seasons and he's probably neck and neck with Ruth for the HR record, and would be well over 3000 hits (conservatively I would put him at 3400), He would easily have the RBI record, almost certainly the runs scored record too. Plus something like 2800 walks....he would have been base over 6000 times. No one would be close.
DiMaggio was a great great hitter, no question; probably underrated by his career numbers. But Ted was greater.
One can argue about the total package...Joe D was no doubt a better baserunner and a better fielder. But Ted was clearly the greater hitter.