It would be highly unusual to get pre-arrival letter so quickly (1 month after applying for sponsorship and days after biometrics) without a completed background check. Should come right before final approval.
i don’t think you need to complete background check for pre-arrival, or is it changed recently?
from what i know, pre-arrival is sent once eligibility is completed and it’s completed very early for me too
edit: i know there is applicant who received pre-arrival before anything (no AOR nor BIL)