The 7 months is the length of time from the first to last months of the 5 overlapping seasons, and there may be ways to have 5 overlapping seasons meeting the criteria that don't quite hit the 7 calendar month duration, but that is at least in the ballpark of the minimum length of time that those conditions must be met. Bigger picture, yes, if conditions are not expected to persist for at least 5 overlapping seasons, then it would not be declared an ENSO event. For example, if we look back at that table, you would see that we had 4 overlapping seasons from OND 2019 through FMA 2020 for which the ONI met the minimum El Nino threshold, but we did not declare El Nino. The reasoning was that the atmosphere did not reflect El Nino conditions, and we did not expect the warmth to persist for long enough (and it didn't). Usually, the persistence and atmospheric coupling criteria go hand in hand - if the ONI duration criterion is not met, it's generally a sign that that the atmosphere and ocean are not cooperating, and we do not see El Nino or La Nina conditions reflected in the tropical atmosphere.