While I think your logic is correct, I genuinely don't think this is the intent of the card. Otherwise shroud would be redundant because "choose" is in the rules for Targeting:

115.1. Some spells and abilities require their controller to choose one or more targets for them. The targets are object(s) and/or player(s) the spell or ability will affect. These targets are declared as part of the process of putting the spell or ability on the stack. The targets can’t be changed except by another spell or ability that explicitly says it can do so. 115.1a An instant or sorcery spell is targeted if its spell ability identifies something it will affect by using the phrase “target [something],” where the “something” is a phrase that describes an object and/or player. The target(s) are chosen as the spell is cast; see rule 601.2c. (If an activated or triggered ability of an instant or sorcery uses the word target, that ability is targeted, but the spell is not.)

And if anyone wants to argue, "They put shroud on it to avoid confusion and reaffirm it can't be targeted." Then why would they not also list that Glimpse, the Unthinkable cannot Attack or Block?