BIMProject files are not readable in Archicad, but in BIMcloud. You can only open the PLN snapshots directly in Archicad. In your case you need a BIMcloud which can open Archicad 26 projects (BIMcloud Basic locked to Archicad 26 or SaaS or paid BIMcloud). You can import them on the BIMcloud Manager's interface and then join to the project the same way as you do to any other Teamwork project.
Sorry I don't fully understand your comment. Aren't your first 2 sentences mutually exclusieve? So the only way to open a snapshot is through the BIMcloud and they can't be opened directly with archicad w/o BIMcloud?