This conclusion has come from their recent discovery of the Tomb of Osiris, which is essentially a large pryamidial hollow underground, and at the base is a huge network of caverns and tunnels that extend to who knows where.
So they now believe these same types of structures are where the actual locations of the kings buried in the pyramids are, in a still sealed underground portion of the pyramid resembling the Tomb of Osiris...
...hmm I wish I had a reference I wouldn't mind reading up on this again myself, but that's what I remember of it, hopefully no false memories

Sincerely,
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