Often, the 100 amp breaker is simply being used as a disconnect for the distribution panel. This would be the case if the feeders from the service equipment were tied to that breaker.
If the 100A breaker was being used as OCPD for another device, then it is obsolete and indicative of repairs/modifications from unqualified persons. The device will trip the 40A breaker at the service equipment before it will trip the 100A breaker.
I wouldn’t say the 100A breaker/disconnect is overrated since the #8 wires are adequately protected in the service panel … but it may not have been tested for terminating the #8 wire which means there may be a potential for a poor connection, which should be checked if you didn’t
Since it is not needed the rating is irrelevant. Also remember that the 100 amp is the max rating, not a minimum rating. It is serving as a redundant shutoff. You will not find a panel with a 40 amp main unless the panel was being backfeed.