You are oversimplifying something that is complex.
Preliminary data suggest that natural immunity from previous strains is below the desired level when it comes to the delta strain.
When compared to vaccine acquired immunity, while less effective than the previous strains, it still is within the desired range of immunity.
there is not a lot of data on the delta variant, but below is from an article in nature named:
Reduced sensitivity of SARS-CoV-2 variant Delta to antibody neutralization
“We examined the sensitivity of this strain to monoclonal antibodies and to antibodies present in sera from individuals who had recovered from COVID-19 (hereafter referred to as convalescent individuals) or who had received a COVID-19 vaccine, and then compared this strain with other strains of SARS-CoV-2. The Delta variant was resistant to neutralization by some anti-NTD and anti-RBD monoclonal antibodies, including bamlanivimab, and these antibodies showed impaired binding to the spike protein. Sera collected from convalescent individuals up to 12 months after the onset of symptoms were fourfold less potent against the Delta variant relative to the Alpha variant (B.1.1.7).”