No such assumption is stated. They decided to honor Gruber's specific requests (stop using the Standard Markup name, kill the domain and don't redirect it, and apology) and, as a courtesy offered him the opportunity to provide feedback on a list of alternative names to use instead of the Standard Markup name.
They never suggested that they took Gruber's silence as equivalent to consent, more that the silence (combined with the general shape of the names he had indicated would probably be acceptable) -- given that they aren't going to cancel the project, and they are going to immediately honor his request to stop using the old name -- leaves them no choice but to choose the new name without further input from Gruber.