J&J (and AZ) is a vectored vaccine, but also delivering the spike protein.

Spike was chosen because it is crucial to the entry of the virus to the cell, and so should be relatively well conserved. Certainly seems to be the case with Delta - but apparently not this new variant.

My understanding is that disease induced immunity can be a bit patchy - if your body develops antibodies to a conserved region, or a wide range, then fine. If to a region that is presented only on the particular virus that infected you, then you may still be vulnerable to further infections.

It has been found that vaccination after infection gives perhaps the best immunity - as if the vaccine focuses the previously too broad immune response somehow.