The report says on page 23:
[…] recent observations from UK Health Security Agency (UKHSA) surveillance data that N antibody levels appear to be lower in individuals who acquire infection following 2 doses of vaccination.
Translation: Recent observations (evaluations) of surveillance data from the UK Health Security Agency (UKHSA) show that N antibody levels appear to be lower in individuals who contract infection after two doses of vaccination.
This simply means that the antibodies produced naturally by the immune system against a virus (specifically a mutation) are noticeably less in those who have been vaccinated twice, which is clearly linked to the vaccination – by the UKHSA itself. The report's conclusion is therefore not surprising: higher antibody levels protect (naturally) better against infection:
