I think it’s the result of compromise. In defining a single oil for all applications, GM was faced with balancing cost against requirements. In this case, some of the performance of 4718M was given up to minimize the cost impact. Unfortunately, following this one oil approach will have some engines operating with oils that fall below the performance level of the oils with which they were originally designed and tested (dexos1 is not better than or equal to GM4718M in every category). But it’s similar to the API’s one-size-for-all approach, where only one performance level is defined (except for the additional “resource conserving” rating).
As an aside the GM4718M spec is probably obsolete, meaning GM will no longer approve any oils to those requirements. So any new formulation SN oil can’t officially meet all GM4718M requirements (which included GM approval/licensing).
The marketing of oil seems to be changing, and I believe we may see more of this type of wording (which isn’t new). I’m aware of one High Mileage oil from a major supplier that makes no reference to any API requirement on the bottle or in the product data sheet. On the website, they simply say that the oil is “suitable for use where API SM, SL and SJ is recommended”.
Perhaps we’ve reached a point where oil suppliers are questioning the value of API approvals in some cases. Confusing? Yes. Confusing by design? Probably not.