Glass half full: We can measure an immune response also in populations where it's more difficult to get a decent one. An immune response means that the immune system actually is taking action against the viral antigen.

Glass half empty: We don't know how the immune response measured relates to protection from or lessening of the infection. This is what the ongoing trial is trying to find out.

This is, IIRC, the second report of successful immunogenicity in older people. There was a (small) cohort of > 65 year old people tested with Pfizer / BioNTech's vaccine in the Phase 1 trial, and even there immune responses were found and measured.