You are right in a sense, because they now include in RESO the Beri-UQAM section and the Guy-Concordia section, which are connected to the underground city only by metro. But the original underground path, the big one under downtown, is now continuous since they build the Quartier International, which connect the two large sections of it (Place des Arts_Convention Center and Peel-McGill).
This new continuous path connects Place des Arts, Complexe Desjardins, the Convention Center, the Quartier International, The stock Exchange tower and Square Victoria, The World Trade Center (including the Hotel Intercontinental and the future Westin now being build), The Bell and National Bank complex, Place Bonaventure, Central Station, the Queen Elizabeth Hotel, 1000 De la Gauchetière, Windsor Station, Bell Center, Place Ville-Marie, Eaton Center, Place de la Cathédrale (including the Bay, Les Ailes de la Mode, etc.), Place Montreal-Trust, The Industrielle center (Paramount cinemas, Simon) and Court Mont-Royal. It's also link to most office towers and hotels on René-Lévesque, DeMaisonneuve and many on Sherbrooke street between University and Peel streets. So in fact, most of Montreal downtown's core is now connected.
The two other sections, Berri-Uqam and Guy-Concordia, are marginal in the RESO and most Montrealers would not even think of them as being part of the underground city. Hope that helps.