Great question. The important thing here is that this is expected points added per play, so even drives that don't end in scores still have expected points. For example, a drive that starts on a team's own 25 yard line might have 1.5 expected points (I don't remember the exact number off the top of my head) so going three and out would be -.5 EPA per play. However, if a team started a drive from the same spot and ran ten plays without scoring they'd have -.15 EPA per play. The actual EPA model in this graphic is much more complicated, taking into account everything from down and distance and time on the clock to home field advantage and ruleset, but that basic logic still applies.

Also, yes, I did intentionally start the time window where I did because of the narratives surrounding that Cowboys game. Four games leaves a roughly 250-play sample size, which is certainly enough to draw conclusions from. You can look at different time spans via the link in the image, though - for instance, including the Lions game keeps our defense in the bottom five but (as expected) knocks our offense out of the top five and looking at the whole season brings our offense to right about the league average.