The Cubs pitchers on the day got 13 strikouts and 2 double plays, leaving 12 of the 27 outs recorded for balls in play. With 16 hits, this means that there were 28 balls put into play. On those balls, the pirates had a batting average of .571! That is extremely rare, average hitters get hits on about 30% of balls put in play. There is a limit to how much you can place a hit, so you can't claim that you are actually that good.
The Pirates weren't tearing the cover off the ball, all of their ridiculous hit total was singles that just found a hole.
Meanwhile the Cubs had 8 hits (the home run doesn't count because it wasn't "in play") for a .286 batting average on balls put in play. The Cubs put a lot of balls in play but didn't get the results the pirates did. These games can happen once in a while, but if the Cubs keep this up they will win most of these games in the future.
Garza did a great job in his Cubs debut.